Fix volume copy for 'virtual' volumes in DotHill
Dot Hill volumes allocated from virtual storage pools require
different handling than volumes allocated from vdisks when
cloning volumes or snapshots.
This change ensures that the correct API calls are used based on
the dothill_backend_type property declared in the cinder.conf.
Change-Id: If2ac15d1d6426948ff439815e85411fbbb95215b
Closes-Bug: #
1489303
Co-Authored-By: Chris Maio <chris.maio@dothill.com>
Co-Authored-By: Lakshman <lakshminarayanat@vedams.com>