mux: simplfy muxes_realize_done

mux_chr_event() already send events to all backends, rename it,
export it, and use it from muxes_realize_done. This should help abstract
away mux implementation.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>