Fix problem of efficient volume copy for migration

Fix problem of efficient volume copy for migration

After introducing commit f586043fa969b9d1dcf4933aacbf615f53691093,
new volume copy method _copy_volume_data() was added into
manager.py. Originally driver.py had this method and it was copied
into manager.py. However new _copy_volume_data() lost efficient
volume copy logic during the reimplementation.

This patch simply add efficient volume copy logic again into new
_copy_volume_data() to fix the problem.

Change-Id: I183cbd2265c1f47c9047818e1d4915c896927280
Closes-Bug: 1491538