Explicitly close VM when the server is shutdown.

tests/conftest.py

@@ -68,7 +68,8 @@ def server(request, loop, port_manager):
 
     def tear_down():
         for module in MODULES:
-            loop.run_until_complete(module.unload())
+            instance = module.instance()
+            loop.run_until_complete(instance.unload())
         srv.close()
         srv.wait_closed()
     request.addfinalizer(tear_down)