This adds a ports= attribute to usb controller XML, like
<controller type='usb' model='nec-xhci' ports='8'/>
This maps to:
qemu -device nec-usb-xhci,p2=8,p3=8
Meaning, 8 ports that support both usb2 and usb3 devices. Gerd
suggested to just expose them as one knob.
https://bugzilla.redhat.com/show_bug.cgi?id=1271408
