This allows to explictily set the physical port where you want to
plug the usb device. Example:
-device usb-tablet,bus=usb.0,port=2
With explicit port addressing qemu can and will not automagically add
USB Hubs. This means that:
(a) You can plug two devices of your choice into the two uhci
root ports.
(b) If you want plug in more that two devices you have to care
about adding a hub yourself.