use restore_file instead of open volume when restoring
use restore_file instead of open volume when restoring

when do _check_restore_vol_size in ceph, cinder-backup
cannot open restored volume image。

Change-Id: I3e093665d37952ab656c205400b227cc073ca472
Closes-bug:#1777588

openstack/cinder,mahak/cinder,mahak/cinder,openstack/cinder