libvirt: Support snapshot creation via libgfapi

Currently, the libvirt driver only handles volume snapshot
creation for file-backed volumes.

This changes allows volume snapshot creation for libgfapi-
attached volumes to work.  (A sufficiently new version of
libvirt is required.)

Implements: blueprint libvirt-volume-snap-network-disk
Closes-Bug: 1288976

Change-Id: Ic6dd861b40b692b25df67c9d5b63fd436c690fde
Merge "libvirt: Support snapshot creation via libgfapi"