Right now, downscript is not invoked reliably. If you execute 'quit' from the
monitor, it won't be invoked.
This fixes that by converting tap to use an exit_notifier to execute the
downscript. In this case, allowing an exit notifier to include state is
critically important for the conversion.
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>