Make I/O more friendly to existing QEMU hardware

Use the serial port for printf() and use the Bochs bios exit
port if the testdev port isn't available.

This unconditionally switches to use the serial port but tries
to use the testdev exit port since that lets you pass an exit
status.

Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
Signed-off-by: Avi Kivity <avi@redhat.com>