Snapshot support for XenAPINFS

Related to blueprint xenapi-storage-manager-nfs

Add support for snapshots, by implementing it with deep copies. Each
snapshot is a volume, so the volume is copied on every snapshot
operation. Snapshot object received the provider_location key, to store
a pointer to the volume.

Change-Id: Iea06246319ce82d5fcfe417419156ec83ef0a895
Format code - eliminate long lines
Add cinder specific bits

Change-Id: I56a9d3e4bcc8833a37bbdbe51ed60c511a35a230
fix bad hacks

Change-Id: Ie79db5e0379ae535594ad79973a80ced348cd72b
Rename migration script

Change-Id: I7ac77249b1f37238e01bcfee7bd0393621827e59
Added tests

Change-Id: I15ed441d58043a3b6e2024209f5b566eeab637fe