Fix HP LeftHand migration with snapshots

Check for snapshots associated with volumes during assisted
migration, and fail if a snapshot is found.

When a volume is cloned on the HP LeftHand array, a snapshot
is created and the cloned volume is created from the snapshot.
The snapshot remains associated with the newly created volume.
Therefore, backend assisted migration should check for volumes
with snapshots and not use native APIs to migrate any volume
that has a snapshot.

Change-Id: I29fc3dbd1f24c01968e7c3d043cdf6b9d1b89ee9
Closes-Bug:#1285829
Merge "Fix HP LeftHand migration with snapshots"