qvm-shutdown with the --wait option checks if the machine
state is 'Halted', but a disposable VM is usually deleted by
the time of the final check, resulting in a non-zero exit code.
This change handles properly disposable VMs, and makes sure
we always output an error message when finishing with a non-zero
exit code.
FixesQubesOS/qubes-issues#5245.
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.
