The method for attaching to update the status of the volume in the
DB is cinder.volume.api.reserve_volume in code and the method for
attaching to finalize the status of the volume and release the
resourece is cinder.volume.api.attach(self, context, volume,
instance_uuid, host_name, mountpoint, mode) in code.
Change-Id: I1af560d5840296d28d6f005b7758b6e75faa727e