Send command line used to start the VM to client

Add a command_line attribute to the VM object with
the command line used to start the VM.

Now /start return the object in order to get
this new attribute. And the HTTP status code
is 200 instead of 204 because 204 disallow body.

Support:
* Qemu
* Dynamips
* IOU

Ref https://github.com/GNS3/gns3-gui/issues/513

tests/handlers/api/test_vpcs.py

@@ -102,7 +102,8 @@ def test_vpcs_start(server, vm):
     with asyncio_patch("gns3server.modules.vpcs.vpcs_vm.VPCSVM.start", return_value=True) as mock:
         response = server.post("/projects/{project_id}/vpcs/vms/{vm_id}/start".format(project_id=vm["project_id"], vm_id=vm["vm_id"]), example=True)
         assert mock.called
-        assert response.status == 204
+        assert response.status == 200
+        assert response.json["name"] == "PC TEST 1"
 
 
 def test_vpcs_stop(server, vm):