Previously, either "--restart" was specified or a prompt was given, with
the prompt being ignored with "--unsafe" in favour of a reboot. This
failed to provide an explicit way to prevent VM restarts with these
commands, which might be desired in some non-interactive situations, and
the interaction of "--unsafe" with this option was an undesired bug.
This is now a complete binary flag with --restart and --no-restart
versions, while still defaulting to a prompt if neither is specified.
This allows full non-interactive control of this option.
