Fix ethernet interfaces tests if eth0 doesn't exists

Also on Windows and MacOS it return error if we try
to use a non existing interfaces.

Fix #230

tests/modules/vpcs/test_vpcs_vm.py

@@ -150,9 +150,9 @@ def test_add_nio_binding_udp(vm):
     assert nio.lport == 4242
 
 
-def test_add_nio_binding_tap(vm):
+def test_add_nio_binding_tap(vm, ethernet_device):
     with patch("gns3server.modules.base_manager.BaseManager._has_privileged_access", return_value=True):
-        nio = VPCS.instance().create_nio(vm.vpcs_path, {"type": "nio_tap", "tap_device": "test"})
+        nio = VPCS.instance().create_nio(vm.vpcs_path, {"type": "nio_tap", "tap_device": ethernet_device})
         vm.port_add_nio_binding(0, nio)
         assert nio.tap_device == "test"