Raise error if using a non linked clone VM twice

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

tests/compute/virtualbox/test_virtualbox_manager.py

@@ -30,7 +30,7 @@ from gns3server.compute.virtualbox.virtualbox_error import VirtualBoxError
 from tests.utils import asyncio_patch
 
 
-@pytest.fixture(scope="module")
+@pytest.fixture
 def manager(port_manager):
     m = VirtualBox.instance()
     m.port_manager = port_manager

tests/compute/virtualbox/test_virtualbox_vm.py

@@ -24,7 +24,7 @@ from gns3server.compute.virtualbox.virtualbox_error import VirtualBoxError
 from gns3server.compute.virtualbox import VirtualBox
 
 
-@pytest.fixture(scope="module")
+@pytest.fixture
 def manager(port_manager):
     m = VirtualBox.instance()
     m.port_manager = port_manager