Description of problem:
virsh domiflist return NULL for the vhostuser interface source

Version-Release number of selected component (if applicable):
libvirt-4.5.0-9.el7.x86_64
qemu-kvm-rhev-2.12.0-15.el7.x86_64


Test steps:
1. Prepare guest with xml:

  <memoryBacking>
    <hugepages>
      <page size='2048' unit='KiB'/>
    </hugepages>
  </memoryBacking>
  ......
  <cpu mode='custom' match='exact' check='full'>
    <model fallback='forbid'>Broadwell</model>
    ......
    <numa>
      <cell id='0' cpus='0-1' memory='1048576' unit='KiB' memAccess='shared'/>
    </numa>
  </cpu>
  ......
    <interface type='vhostuser'>
      <mac address='52:54:00:93:51:dd'/>
      <source type='unix' path='/var/run/openvswitch/vhost-user1' mode='client'/>
      <model type='virtio'/>
      <alias name='net0'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x08' function='0x0'/>
    </interface>

2. Start the guests with vhostuser interface

# virsh start vhostuser1
Domain vhostuser1 started

# virsh list --all
 Id    Name                           State
----------------------------------------------------
 25    vhostuser1                     running

3. Check the virsh domiflist: no source is listed

# virsh domiflist vhostuser1
Interface  Type       Source     Model       MAC
-------------------------------------------------------
vhost-user1 vhostuser  -          virtio      52:54:00:93:51:dd

Actual results:
In step3, the virsh domiflist return NULL for source

Expected results:
In step3, the virsh domiflist return the vhostuser source