un-register kbd driver in case of USB kbd unplug.

If a USB keyboard is unplugged, the keyboard eventhandler is never
removed, and events will continue to be passed through to the device,
causing crashes or memory corruption.

Signed-off-by: Jes Sorensen <Jes.Sorensen@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
