cinder-backup ceph driver restore failure

[Edward Hope-Morley]

The following scenario currently fails with the ceph backup driver:

 1. restore to rbd (i.e. incremental/differential restore is allowed if it that was how the backup was done)
 2. restore destination is the same volume as the backup source

This currently breaks because the restore point is not provide by _diff_restore_allowed()

(NOTE: patch https://review.openstack.org/#/c/51900/ is now being broken up and is the source of the patch to fix this)

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/71641

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/71641
Committed: https://git.openstack.org/cgit/openstack/cinder/commit/?id=4d1d9349fed815b37b0675d6f3936243a3909125
Submitter: Jenkins
Branch: master

commit 4d1d9349fed815b37b0675d6f3936243a3909125
Author: Edward Hope-Morley <email address hidden>
Date: Thu Feb 6 13:06:44 2014 +0000

    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