mux: fix ctrl-a b again

Commit fb5e19d2e1 originally fixed the
regression, but was inadvertently broken again in merge commit
2d6752d38d.

Fixes:
https://bugs.launchpad.net/qemu/+bug/1654137

Cc: qemu-stable@nongnu.org
Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-id: 20180416181844.7851-1-marcandre.lureau@redhat.com
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

chardev/char-mux.c

@@ -304,6 +304,7 @@ void mux_set_focus(Chardev *chr, int focus)
     }
 
     d->focus = focus;
+    chr->be = d->backends[focus];
     mux_chr_send_event(d, d->focus, CHR_EVENT_MUX_IN);
 }