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

gns3server/modules/base_vm.py

@@ -58,6 +58,7 @@ class BaseVM:
         self._hw_virtualization = False
         self._ubridge_hypervisor = None
         self._vm_status = "stopped"
+        self._command_line = ""
 
         if self._console is not None:
             if console_type == "vnc":
@@ -94,6 +95,17 @@ class BaseVM:
         self._vm_status = status
         self._project.emit("vm.{}".format(status), self)
 
+    @property
+    def command_line(self):
+        """Return command used to start the VM"""
+
+        return self._command_line
+
+    @command_line.setter
+    def command_line(self, command_line):
+
+        self._command_line = command_line
+
     @property
     def project(self):
         """