(
  source only properties:
  {
    "driver": "file",
    "filename": "/var/lib/libvirt/images/a"
  }
  backing store string:
  /var/lib/libvirt/images/a
)
(
  source only properties:
  {
    "driver": "file",
    "filename": "/var/lib/libvirt/images/b"
  }
  backing store string:
  /var/lib/libvirt/images/b
)