Description of problem:

https://www.redhat.com/archives/libvir-list/2018-June/msg00490.html

If you have a reference to a relative path for a socket,
these no longer work.  They used to in older versions of libvirt,
and they still work on certain architectures (for example, x86_64
still works, but i686 fails).

libvirt should either reject them if they are wrong, or be fixed
so that it works like it used to.

If you decide to reject them, please clone this bug for
libguestfs because we will need to fix the paths that we
pass to libvirt.

Version-Release number of selected component (if applicable):

libvirt 4.4.0-1.fc29

How reproducible:

Unknown, only on certain architectures at certain times.
I was only able to reliably reproduce on i686 in a Koji
build using qemu:///session.

Steps to Reproduce:

1. Start nbdkit serving over a local Unix domain socket in the
current directory:

rm -f my.sock
nbdkit -f -v -U my.sock example1

2. Create a guest which has a disk like:

  <disk device="disk" type="network">
    <source protocol="nbd">
      <host transport="unix" socket="my.sock"/>
    </source>
    <target dev="sda" bus="scsi"/>
    <driver name="qemu" type="raw" cache="writeback"/>
  </disk>

3. Start the guest under qemu:///session

4. Fails with:

  internal error: process exited while connecting to monitor: 2018-06-06T17:02:5
4.450507Z qemu-system-i386: -drive
file=nbd+unix://?socket=my.sock,format=raw,if=none,id=drive-scsi0-0-0-0,cache=w
riteback: Failed to connect socket my.sock: No such file or directory [code=1
int1=-1]