'``compute_instance_id`` raises an error when it cannot return a valid
instance ID.'

def test_missing_instance_id(self):

    root_path = None
    api = LoopbackBlockDeviceAPI(root_path, compute_instance_id=None)
    e = self.assertRaises(UnknownInstanceID, api.compute_instance_id)
    self.assertEqual(('Could    not    find    valid    instance    ID    for    %r' % (api,)), str(e))
