Can't use efficient volume copy for generic volume migration

[Mitsuhiro Tanino]

After introducing 'Add support for file I/O volume migration' via https://review.openstack.org/#/c/187270/ , new volume copy method _copy_volume_data() was added into manager.py and the generic volume migration was changed to use this new function.

However this method does not have efficient volume copy logic which was implemented in copy_data_volume() of driver.py.(via https://review.openstack.org/#/c/183701/)

We should add same check logic into _copy_volume_data() to support efficient volume copy.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/219867

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/219867
Committed: https://git.openstack.org/cgit/openstack/cinder/commit/?id=05047c91674453a4ae209a7e29f399853d1dd10c
Submitter: Jenkins
Branch: master

commit 05047c91674453a4ae209a7e29f399853d1dd10c
Author: Mitsuhiro Tanino <email address hidden>
Date: Wed Sep 2 14:33:18 2015 -0400

    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