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