usb: make usb_create_simple catch and pass up errors.

Use qdev_init() instead of qdev_init_nofail(), usb device initialization
can fail, most common case being port and device speed mismatch.  Handle
failures correctly and pass up NULL pointers then.

Also fixup usb_create_simple() callers (only one was buggy) to properly
check for NULL pointers before referncing the usb_create_simple() return
value.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
