udev rule to populate /dev/vboxusb

This fixes NixOS/nixpkgs#457. 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.

  This patch adds those rules.