So that devices can be attached to hubs. Example, to attach to first
port of a usb-hub on port 1.
<hub type='usb'>
<address type='usb' bus='0' port='1'/>
</hub>
<input type='mouse' type='usb'>
<address type='usb' bus='0' port='1.1'/>
</hub>
also add a test entry