Snapshot support for XenAPINFS

Related to blueprint xenapinfs-snapshots

Add support for snapshots, by implementing it with deep copies. Each
snapshot is a copy of the volume. Snapshot object has a new,
provider_location key, to store volume parameters.

Change-Id: Iea06246319ce82d5fcfe417419156ec83ef0a895
Merge "Snapshot support for XenAPINFS"