[libvirt] disable usb?

Hi, Is there some way to disable usb altogether? libvirt used to just pass in '-usb'. With the arrival of usb2 support that changed into '-device uhci,...'. Problem is that this breaks with several machine types such as isapc. '-usb' is silently ignored in case the machine type can't handle usb. '-device uhci,...' leads to an error message though and the guest doesn't start. /me tried "<controller type='usb' model='none'/>" which didn't work. Just removing the controller from the xml doesn't work too, it gets automagically readded. Suggestions? thanks, Gerd