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.
When some VM timeout on shutdown, the tool will try to kill all of them,
but at this point some of them may be already powered off (not all
hanged during shutdown, but only some). Handle this
situation instead of crashing. And add appropriate test.
