[libvirt-users] Programmatically force shutdown a guest: possible?

I am looking at the shutdown method, but if the guest system is a desktop system, like for example ubuntu, it just has the effect to show a box prompting the user for a shutdown/reboot/ and such. I could enter the guest and change this default behaviour and it actually works..but I'd like for a way to send a shutdown command without doing so. Is it possible?