Move live migration callbacks to BDM objects

This patch moves the two live migration callbacks _post_live_migration
and _rollback_live_migration to use BDM objects.

As this is the last code that was using _get_instance_volume_bdms helper
method, this patch removes it.

Part of blueprint: icehouse-objects

Change-Id: I904d8b5b58d0ee4ba5a5d836fe3f8a236793f33b
Merge "Move live migration callbacks to BDM objects"