* Add USBBus.
 * Add USBDeviceInfo, move device callbacks here.
 * Add usb-qdev helper functions.
 * Switch drivers to qdev.

TODO:
 * make the rest of qemu aware of usb busses and kill the FIXMEs
   added by this patch.
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>