Move compute's _get_instance_volume_block_device_info to BDM objects

This patch makes the compute manager's
_get_instance_volume_block_device_info method use objects internally.
This method is being called in a number of places, which makes this a
convenient place to start introducing BDM objects in compute.

Also moves the tests that needed fixing to mock.

Part of blueprint: clean-up-legacy-block-device-mapping
Part of blueprint: icehouse-objects

Change-Id: Ied2dcb860e2ba375117bab341a56b9fea516cffe