VirtualBox: udev rule to populate /dev/vboxusb.

After Linux 3.2(?), /proc/bus/usb (and usbfs (or usbdevfs?)) went away,
leaving virtualbox no way to determine what USB devices were connected
to the system. The solution was to add some virtualbox specific udev
rules to populate /dev/vboxusb with what was in /proc/bus/usb before.

Patch contributed by Jack Cummings.