Do not allow reboot/shutdown on a guest session on a webkiosk host
(all webkiosk host sessions are guest sessions, but we do an extra
check).  This does not actually prevent users from reboot or
shutting down webkiosk hosts, simply breaks the functionality
from webmenu (whose user interface should be modified regarding this).

This is a quick hack that might be taken away when a more proper
solution is in place.  Perhaps that might include dbus-configuration
change and webmenu interface change.