Raise error if using a non linked clone VM twice

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

tests/compute/vmware/test_vmware_manager.py

@@ -29,7 +29,7 @@ from gns3server.compute.vmware import VMware
 from tests.utils import asyncio_patch
 
 
-@pytest.fixture(scope="module")
+@pytest.fixture
 def manager(port_manager):
     m = VMware.instance()
     m.port_manager = port_manager

tests/compute/vmware/test_vmware_vm.py

@@ -24,7 +24,7 @@ from gns3server.compute.vmware.vmware_error import VMwareError
 from gns3server.compute.vmware import VMware
 
 
-@pytest.fixture(scope="module")
+@pytest.fixture
 def manager(port_manager):
     m = VMware.instance()
     m.port_manager = port_manager