Unimplemented pause should not change vm state on PowerVM

The instance of PowerVM does not support pause, but
after pause the instance, the state change to PAUSED,
it's unreasonable.

Change-Id: I4fb184388c37ea8708cdacc173dc1b2dbd4d1597
Fixes: bug #1202082

nova/virt/powervm/driver.py

@@ -178,11 +178,13 @@ def snapshot(self, context, instance, image_id, update_task_state):
 
     def pause(self, instance):
         """Pause the specified instance."""
-        pass
+        msg = _("pause is not supported for PowerVM")
+        raise NotImplementedError(msg)
 
     def unpause(self, instance):
         """Unpause paused VM instance."""
-        pass
+        msg = _("unpause is not supported for PowerVM")
+        raise NotImplementedError(msg)
 
     def suspend(self, instance):
         """suspend the specified instance."""