The bug is that virt-v2v chokes on a blank disk.  Here's an
easier way to reproduce it that doesn't require root or VMware:

$ virt-builder centos-6
$ rm -f blank.img
$ truncate -s 1G blank.img

Put the following XML into a file called 'test.xml':

<domain type='kvm'>
  <name>test</name>
  <memory>1048576</memory>
  <vcpu>2</vcpu>
  <os>
    <type>hvm</type>
    <boot dev='hd'/>
  </os>
  <features>
    <acpi/>
    <apic/>
    <pae/>
  </features>
  <devices>
    <disk type='file' device='disk'>
      <driver name='qemu' type='raw'/>
      <source file='centos-6.img'/>
      <target dev='hda' bus='ide'/>
    </disk>
    <disk type='file' device='disk'>
      <driver name='qemu' type='raw'/>
      <source file='blank.img'/>
      <target dev='hdb' bus='ide'/>
    </disk>
    <interface type='network'>
      <mac address='52:54:00:01:02:03'/>
      <source network='default'/>
      <model type='rtl8139'/>
    </interface>
  </devices>
</domain>

Then run virt-v2v:

$ virt-v2v -i libvirtxml test.xml -o null
[   0.0] Opening the source -i libvirtxml test.xml
[   0.0] Creating an overlay to protect the source from being modified
[   0.6] Opening the overlay
[  11.7] Initializing the target -o null
[  11.7] Inspecting the overlay
virt-v2v: error: libguestfs error: part_get_parttype: unknown signature, 
                              of the output: 
BYT;                                        

If reporting bugs, run virt-v2v with debugging enabled and include the 
complete output:

  virt-v2v -v -x [...]

The reason the error message is oddly corrupted, is because of
a further and unrelated bug in parted.  I filed that separately
(bug 1232241).

Upstream fix:
https://github.com/libguestfs/libguestfs/commit/4c73d1d4f142c6f6211c963beea68773e11fd3ef

I can reproduce this issue with old package in a simple way:

virt-v2v-1.28.1-1.38.el7.x86_64
libguestfs-1.28.1-1.38.el7.x86_64
libvirt-1.2.16-1.el7.x86_64

Steps to reproduce:

1. On you local host, there is a health guest.
# virsh list --all
 Id    Name                           State
----------------------------------------------------
 -     rhel6.7-snap5-clone1           shut off

2. Create a empty disk and add it to this guest.
# truncate -s 1G  /tmp/blank.img
# virsh dumpxml rhel6.7-snap5-clone1
...
    <disk type='file' device='disk'>
      <driver name='qemu' type='raw'/>
      <source file='/var/lib/libvirt/images/rhel6.7-snap5-clone1.img'/>
      <target dev='vda' bus='virtio'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x07' function='0x0'/>
    </disk>
    <disk type='file' device='disk'>
      <driver name='qemu' type='raw'/>
      <source file='/tmp/blank.img'/>
      <target dev='vdb' bus='virtio'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x09' function='0x0'/>
    </disk>
...

3. Save guest xml file then undefine guest.
# virsh dumpxml rhel6.7-snap5-clone1 >rhel6.7-snap5-clone1.xml
# virsh undefine rhel6.7-snap5-clone1 

4. Run virt-v2v cmd:
# virt-v2v -i libvirtxml rhel6.7-snap5-clone1.xml -o null
[   0.0] Opening the source -i libvirtxml rhel6.7-snap5-clone1.xml
[   0.0] Creating an overlay to protect the source from being modified
[   1.0] Opening the overlay
[ 163.0] Initializing the target -o null
[ 163.0] Inspecting the overlay
virt-v2v: error: libguestfs error: part_get_parttype: unknown signature, 
                              of the output: 
BYT;                                        

If reporting bugs, run virt-v2v with debugging enabled and include the 
complete output:

  virt-v2v -v -x [...]


Then try to verify this bug with new build:
virt-v2v-1.28.1-1.40.el7.x86_64
libguestfs-1.28.1-1.40.el7.x86_64
libvirt-1.2.16-1.el7.x86_64

Steps as above:
1. 
# virt-v2v -i libvirtxml rhel6.7-snap5-clone1.xml -o null
[   0.0] Opening the source -i libvirtxml rhel6.7-snap5-clone1.xml
[   0.0] Creating an overlay to protect the source from being modified
[   0.0] Opening the overlay
[ 107.0] Initializing the target -o null
[ 107.0] Inspecting the overlay
[ 118.0] Checking for sufficient free disk space in the guest
[ 118.0] Estimating space required on target for each disk
[ 118.0] Converting Red Hat Enterprise Linux Server release 6.7 Beta (Santiago) to run on KVM
virt-v2v: This guest has virtio drivers installed.
[ 161.0] Mapping filesystem data to avoid copying unused and blank areas
[ 161.0] Closing the overlay
[ 161.0] Copying disk 1/2 to /var/tmp/null.tp85vi/sda (raw)
    (100.00/100%)
[ 235.0] Copying disk 2/2 to /var/tmp/null.tp85vi/sdb (raw)
    (100.00/100%)
[ 237.0] Creating output metadata
[ 237.0] Finishing off

2. Always test again with guest on Comment 0:
# virt-v2v -ic  vpx://root.72.112/data/10.66.72.49/?no_verify=1 test-juzhou  --password-file /tmp/passwd2   
[   0.0] Opening the source -i libvirt -ic vpx://root.72.112/data/10.66.72.49/?no_verify=1 test-juzhou
[   1.0] Creating an overlay to protect the source from being modified
[   2.0] Opening the overlay
[ 106.0] Initializing the target -o libvirt -os default
[ 106.0] Inspecting the overlay
[ 163.0] Checking for sufficient free disk space in the guest
[ 163.0] Estimating space required on target for each disk
[ 163.0] Converting Red Hat Enterprise Linux Server release 6.6 (Santiago) to run on KVM
virt-v2v: This guest has virtio drivers installed.
[ 530.0] Mapping filesystem data to avoid copying unused and blank areas
[ 532.0] Closing the overlay
[ 532.0] Copying disk 1/2 to /var/lib/libvirt/images/test-juzhou-sda (raw)
    (100.00/100%)
[ 598.0] Copying disk 2/2 to /var/lib/libvirt/images/test-juzhou-sdb (raw)
    (100.00/100%)
[ 618.0] Creating output metadata
Pool default refreshed

Domain test-juzhou defined from /tmp/v2vlibvirt03b511.xml

[ 620.0] Finishing off

Since virt-v2v can convert guest with no error, move this bug from ON_QA  to VERIFIED.

Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.

For information on the advisory, and where to find the updated
files, follow the link below.

If the solution does not work for you, open a new bug report.

https://rhn.redhat.com/errata/RHBA-2015-2183.html