Fix restore point if backup base is diff-format in ceph

Fix restore point if backup base is diff-format in ceph

In Ceph when backup base is diff-format and use this backup
to restore volumes not rbd, need to find out the correct restore
point.

This is fixed in cinder/backup/drivers/ceph.py _diff_restore_allowed.

Before the change: when backup is diff-format, and destination volume
is not rbd, the function returns restore_point as None. As a result,
backup restore uses wrong image/snapshot to do restore.

With the change, we always get the right restore point, regardless
of the kind of restore we'll do differential or not.

Change-Id: I006c05d246f59fc6aff597543bedc68589d37576
Closes-Bug: #1472088