Description of problem:
unprivileged user failed to start domain

Version-Release number of selected component (if applicable):
libvirt-1.3.5-1.el7.ppc64le
qemu-kvm-rhev-2.6.0-11.el7.ppc64le
kernel-3.10.0-327.el7.ppc64le

How reproducible:
always

Steps to Reproduce:
1.
# su - new_user
$ virsh list --all
 Id    Name                           State
----------------------------------------------------
 -     avocado-vt-vm1                 shut off

$ virsh dumpxml avocado-vt-vm1
<domain type='kvm'>
  <name>avocado-vt-vm1</name>
  <uuid>1c2363d5-90da-4f59-b1f8-25fbb4bec2d8</uuid>
  <memory unit='KiB'>1048576</memory>
  <currentMemory unit='KiB'>1048576</currentMemory>
  <vcpu placement='static'>2</vcpu>
  <os>
    <type arch='ppc64le' machine='pseries-rhel7.3.0'>hvm</type>
    <boot dev='hd'/>
  </os>
  <clock offset='utc'/>
  <on_poweroff>destroy</on_poweroff>
  <on_reboot>restart</on_reboot>
  <on_crash>restart</on_crash>
  <devices>
    <emulator>/usr/libexec/qemu-kvm</emulator>
    <disk type='file' device='disk'>
      <driver name='qemu' type='qcow2'/>
      <source file='/tmp/autotest.img'/>
      <target dev='vda' bus='virtio'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x04' function='0x0'/>
    </disk>
    <controller type='usb' index='0'>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x02' function='0x0'/>
    </controller>
    <controller type='pci' index='0' model='pci-root'/>
    <controller type='virtio-serial' index='0'>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x03' function='0x0'/>
    </controller>
    <interface type='bridge'>
      <mac address='52:54:00:f4:85:91'/>
      <source bridge='virbr0'/>
      <model type='virtio'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x01' function='0x0'/>
    </interface>
    <serial type='pty'>
      <target port='0'/>
      <address type='spapr-vio' reg='0x30000000'/>
    </serial>
    <console type='pty'>
      <target type='serial' port='0'/>
      <address type='spapr-vio' reg='0x30000000'/>
    </console>
    <input type='keyboard' bus='usb'/>
    <input type='mouse' bus='usb'/>
    <graphics type='vnc' port='-1' autoport='yes'>
      <listen type='address'/>
    </graphics>
    <video>
      <model type='vga' vram='16384' heads='1' primary='yes'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x06' function='0x0'/>
    </video>
    <memballoon model='virtio'>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x05' function='0x0'/>
    </memballoon>
    <panic model='pseries'/>
  </devices>
</domain>

2. start domain
$ virsh start avocado-vt-vm1
error: Failed to start domain avocado-vt-vm1
error: Failed to connect socket to '/home/new_user/.cache/libvirt/virtlogd-sock': Connection refused

some problem with virtlogd, as on x86_64 this works

3. start virtlogd under this user to workaround virtlogd problem
$ virtlogd --daemon

$ ps aux|grep virtlogd
new_user 143900  0.0  0.0 209344 12032 ?        Sl   05:08   0:00 virtlogd --daemon
root     144096  0.0  0.0 209344 17024 ?        Ssl  05:10   0:00 /usr/sbin/virtlogd
new_user 146793  0.0  0.0 111040  2816 pts/0    S+   05:48   0:00 grep --color=auto virtlogd

4. start domain again
$ virsh start avocado-vt-vm1
error: Failed to start domain avocado-vt-vm1
error: internal error: /usr/libexec/qemu-bridge-helper --use-vnet --br=virbr0 --fd=24: failed to communicate with bridge helper: Transport endpoint is not connected
stderr=libvirt:  error : internal error: cannot apply process capabilities -1

Check virbr0
# brctl show
bridge name	bridge id		STP enabled	interfaces
virbr0		8000.5254006a8738	yes		virbr0-nic

# virsh net-list --all
 Name                 State      Autostart     Persistent
----------------------------------------------------------
 default              active     yes           yes

# cat /etc/qemu-kvm/bridge.conf
allow virbr0

# virsh net-dumpxml default
<network>
  <name>default</name>
  <uuid>3fd1334e-5d47-4058-a7c8-ec08c8949f79</uuid>
  <forward mode='nat'>
    <nat>
      <port start='1024' end='65535'/>
    </nat>
  </forward>
  <bridge name='virbr0' stp='on' delay='0'/>
  <mac address='52:54:00:6a:87:38'/>
  <ip address='192.168.122.1' netmask='255.255.255.0'>
    <dhcp>
      <range start='192.168.122.2' end='192.168.122.254'/>
    </dhcp>
  </ip>
</network>


Actual results:
failed to start

Expected results:
succeed

Additional info: