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/modules/vpcs/test_vpcs_vm.py

@@ -107,6 +107,7 @@ def test_start(loop, vm):
                                               '-t',
                                               '127.0.0.1')
             assert vm.is_running()
+            assert vm.command_line == ' '.join(mock_exec.call_args[0])
     (action, event) = queue.get_nowait()
     assert action == "vm.started"
     assert event == vm