Remove _instance_update usage in _default_block_device_names

We have an instance object in _default_block_device_names so
we can just call instance.save() directly.  We don't need to
update resource tracker stats so there is no need to call
_instance_update in this method.

Part of blueprint compute-manager-objects-juno

Change-Id: Ia8faa67a36ff31a3ba526770f5d8a0361326ffc7
Merge "Remove _instance_update usage in _default_block_device_names"