tap: use an exit notifier to call down_script

tap: use an exit notifier to call down_script

We would like to move back net_cleanup() at the end of main function,
like it used to be until f30dbae63a46f23116715dff8d130c, but minimum
tap cleanup is necessary regarless at exit() time. Use an exit notifier
to call TAP down_script. If net_cleanup() is called first, then remove
the exit notifier as it will become a dangling pointer otherwise.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20160711144847.16651-1-marcandre.lureau@redhat.com>
Reviewed-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>