Move is_volume_backed_instance to new style BDM

Compute API is_volume_backed_instance used to rely on matching the
device name to the root_device_name field of the instance which had
several problems and edge cases.

Now that we have moved to the new BDM format that has the boot_index
and destination_type field for each block device - it is pretty
staightforward to implement.

Change-Id: Ia2c235f8bce577ed4ac6a480a4a051d159a5fad7
blueprint: clean-up-legacy-block-device-mapping
Merge "Move is_volume_backed_instance to new style BDM"