No relevant message when stop a stopped VM

When user tries to stop a stopped VM, there is no need to stop it again,
and We need return it's vm_state.

Fixes bug 1181934

Change-Id: I71772dfcead440cb49b6ccdbb4e1a891475bb75e
(cherry picked from commit 62ca378)

nova/compute/api.py

@@ -1280,7 +1280,7 @@ def force_delete(self, context, instance):
     @check_instance_lock
     @check_instance_host
     @check_instance_state(vm_state=[vm_states.ACTIVE, vm_states.RESCUED,
-                                    vm_states.ERROR, vm_states.STOPPED],
+                                    vm_states.ERROR],
                           task_state=[None])
     def stop(self, context, instance, do_cast=True):
         """Stop an instance."""

nova/tests/compute/test_compute.py

@@ -4679,6 +4679,16 @@ def test_stop(self):
 
         db.instance_destroy(self.context, instance['uuid'])
 
+    def test_stop_a_stopped_inst(self):
+        instance = jsonutils.to_primitive(self._create_fake_instance(
+                                          {'vm_state': vm_states.STOPPED}))
+
+        self.assertRaises(exception.InstanceInvalidState,
+                          self.compute_api.stop,
+                          self.context, instance)
+
+        db.instance_destroy(self.context, instance['uuid'])
+
     def test_stop_no_host(self):
         instance = self._create_fake_instance(params={'host': ''})