usb: add port property.

usb: add port property.

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.

Plugging a hub works this way:

  -device usb-hub,bus=usb.0,port=1

Use this to add a device to the hub:

  -device usb-tablet,bus=usb.0,port=1.1

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>