reduce network down time during live-migration

Called unplug_vifs() method before post_live_migration_to_destination call
because floating ip address will not work until the vifs are unplugged
from the source compute node.

Added new method post_live_migration_at_source in virt driver and
implemented it in libvirt driver to unplug the vifs at source. Other
drivers will raise NotImplementedError.
Added new keyword argument destroy_vifs to cleanup() method to verify
if unplug_vifs() is already called then do not call it again.

Closes-Bug: #1284996
Change-Id: Ida3ed3bef77239e5a0fdbf8866e8a4421d4f8222
Merge "reduce network down time during live-migration"