Move usb code from vl.c to usb-bus.c and make it use the new data
structures added by qdev conversion.  qemu usb core should be able
to handle multiple USB busses just fine now (untested though).

Kill some usb_*_init() legacy functions, use usb_create_simple()
instead.

Kill some FIXMEs added by the first qdev/usb patch.
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>