Fix cinder-backup volume restore with ceph driver

Restore operations currently break if restoring to
the backup source volume, that volume is an rbd and
the associated backup was incremental.

Change-Id: Ieafe6ab2d1a15cad2b534a3aab0df29eb8591306
Closes-Bug: 1276977
Merge "Fix cinder-backup volume restore with ceph driver"