Added this entry to the file:

<menuitem priority="5" name="RebootX">
<callback service="com.nokia.xterm" path="/com/nokia/xterm" interface="com.nokia.xterm" method="run_command" bus="session" autostart="true">
<argument type="string">/sbin/reboot</argument>
</callback>
</menuitem>


There is now a button RebootX but all is seems to do is start an xterm window, close it and start another one. I think the command needs root privileges...