new cmd line syntax:  you can add mux=1 to any chardev to enable muxing,
then attach it multiple times, like this:

	-chardev pty,name=mux,mux=on
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>