libvirt: parse disk backing chains from domain XML

Add support for parsing <backingStore> XML elements in the libvirt
domain XML description for disks.

This is being added to libvirt in order to support blockjob operations
for network-attached disks.

Implements: blueprint libvirt-volume-snap-network-disk

Change-Id: I370c9e8d6a1b1f66e385f009f897816f2e705a36
Merge "libvirt: parse disk backing chains from domain XML"