monitor: fix muxing make the mux driver send mux_in and mux_out events when switching focus while hooking up more handlers. stop using CharDriverState->focus in monitor.c, track state using the mux events instead. This also removes the implicit assumtion that a muxed monitor allways has mux channel 0. Signed-off-by: Gerd Hoffmann <kraxel@redhat.com> Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>