usb-bot: Mark device as non hotpluggable

usb-bot creates SCSI bus and immediately makes it
non hotpluggable which was making not possible to
hotplug usb-bot since QEMU would abort at
bus_add_child(scsi-hd) time when usb-bot is
realized.

Mark usb-bot as not hotpluggable so that attempt
to hotplug it would error out even before it gets
to device initialization point.

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>