When creating an USB device the old way, there is no way to specify the
target bus. Thus the warning issued by usb_create makes no sense and
rather confuses our users.

Resolve this by passing a bus reference to the usbdevice_init handler
and letting those handlers forward it to usb_create.
Signed-off-by: NJan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>