GlusterFS: Use correct source file in copy_volume_from_snapshot

The GlusterFS driver's copy_volume_from_snapshot was previously
not parsing the snapshot information to determine which backing
file to copy from, instead copying from the active file at all
times.  This would result in the wrong data being supplied when
a volume is cloned from a snapshot.

Closes-Bug: 1222907

Change-Id: Ib829ca1a6812b61845f3b2eb9c5507779fa6ec15