Fix a crash in VirtualBox vm creation

Fix #138

gns3server/handlers/api/virtualbox_handler.py

@@ -79,8 +79,9 @@ class VirtualBoxHandler:
                 yield from vm.set_ram(ram)
 
         for name, value in request.json.items():
-            if hasattr(vm, name) and getattr(vm, name) != value:
-                setattr(vm, name, value)
+            if name != "vm_id":
+                if hasattr(vm, name) and getattr(vm, name) != value:
+                    setattr(vm, name, value)
 
         response.set_status(201)
         response.json(vm)