qga/commands-posix: 'guest-shutdown' for Solaris

qga/commands-posix: 'guest-shutdown' for Solaris

On Solaris, instead of the -P, -H, and -r flags, we need to provide
the target init state to the 'shutdown' command: state 5 is poweroff,
0 is halt, and 6 is reboot. We also need to pass -g0 to avoid the
default 60-second delay, and -y to avoid a confirmation prompt.

Implement this logic under an #ifdef CONFIG_SOLARIS, so the
'guest-shutdown' command works properly on Solaris.

Signed-off-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20220426195526.7699-6-adeason@sinenomine.net>