zap serial_monitor_mux

zap serial_monitor_mux

The logic in this code obviously predates the multiple monitor
capability of qemu and looks increasingly silly these days.

I think the intention of this piece of code is to get a reasonable
default for the -nographic case: have monitor and serial line muxed
on stdio.

With the new default_serial and default_monitor variables we have now
doing just that became much easier ;)

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
(cherry picked from commit e1c09175bc00dd8dfb2ad1b26e1858dcdc109b59)