This looks correct.
I think I know what happened. There is a check if default_dispvm is none, with a comment user must have changed it manually - in which case the above default does not apply. But that may also happen if the global default dispvm is set to none - is it? Is it better if you change the global default to something else?
@arraybolt3 should it maybe check for vm.property_is_default("default_dispvm") instead?