It wasn't possible to use QubesArgumentParser(vmname_nargs=...) for
optional domain list - the option forced usage of either --all or
explicit domain list.
Very few calls at client side really needs VM class name. So, even in
non-blind mode use just QubesVM class, to avoid strange cases depending
on blind mode being enabled or not. Then, have VM class name in 'klass'
property. If known at object creation time, cache it, otherwise query
qubesd at first access.
