'Destroy a block device mapping in API cells by device name
or volume_id.  device_name or volume_id can be None, but not both.'

def bdm_destroy_at_top(self, message, instance_uuid, device_name, volume_id):

    if (not self._at_the_top()):
        return
    if device_name:
        self.db.block_device_mapping_destroy_by_instance_and_device(message.ctxt, instance_uuid, device_name)
    elif volume_id:
        self.db.block_device_mapping_destroy_by_instance_and_volume(message.ctxt, instance_uuid, volume_id)
