Raise error if using a non linked clone VM twice

Fix https://github.com/GNS3/gns3-gui/issues/1593

tests/compute/vpcs/test_vpcs_vm.py

@@ -31,7 +31,7 @@ from gns3server.compute.vpcs import VPCS
 from gns3server.compute.notification_manager import NotificationManager
 
 
-@pytest.fixture(scope="module")
+@pytest.fixture
 def manager(port_manager):
     m = VPCS.instance()
     m.port_manager = port_manager