This simplifies removing a backend for a frontend user (no need to
retrieve the associated driver and separate delete call etc).
NB: many frontends have questionable handling of ending a chardev. They
should probably delete the backend to prevent broken reusage.
Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>