migrate_data attribute in post_live_migration_at_destination

It would be an advantage if the _post_live_migration(...) method from nova/compute/manager.py calls a post_live_migration_at_destination(..) method by passing migrate_data as an attribute.

Also if the post_live_migration_at_destination nmethod in nova/compute/rpcapi.py accepts the migrate_data.py.

To make it backward compatible we can **kwargs in the method declaration.