usb: Improve -usbdevice error reporting a bit

Most LegacyUSBFactory usbdevice_init() methods realize with
qdev_init_nofail(), even though their caller usbdevice_create() can
handle failure.  Okay if it really can't fail (I didn't check), but
somewhat brittle.

usb_msd_init() and usb_bt_init() call qdev_init().  The latter
additionally reports an error when qdev_init() fails.

Realization failure produces multiple error reports: a specific one
from qdev_init(), and generic ones from usb_bt_init(),
usb_create_simple(), usbdevice_create() and usb_parse().

Remove realization from the usbdevice_init() methods.  Realize in
usbdevice_create(), and produce exactly one error message there.  You
still get another one from usb_parse().

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