Add backend assisted volume migration to Oracle ZFSSA drivers

Currently, OpenStack Cinder provides generic volume migration functionality. This
generic volume migration copies blocks of data from the source volume to a
destination volume created on another backend using 'dd' utility.
This makes the process slow and it also uses resources on the Cinder host while
copying data.
Oracle ZFS Storage Appliance(ZFSSA) drivers can utilize some of the data migration
services that ZFSSA provides when the volume migration happens from one backend
configured with Oracle ZFSSA to another backend also configured with Oracle ZFSSA.
This type of volume migration will bypass the cinder host and will directly
transfer the volume to the destination backend.