Interesting, could you shed a light on what exact XML is needed ?
http://libvirt.org/formatdomain.html#elementsGraphics http://libvirt.org/formatdomain.html#elementsVideo
You need to set the video type to qxl and the graphical type to spice ... then set the appropriate attributes on the element port, sport, etc etc
There should be more info on the Fedora sites as well.
James