pc: make vgabios exit port more useful

We've always listened on port 501 for vgabios panic messages.  In the entire
time I've worked on QEMU, I've never actually seen a vgabios panic message :-)

If we change the semantics of this port a little bit, it makes it possible to
use it for more interesting use-cases.  I chose this approach instead of adding
a new I/O port because it avoids having a guest visible change.

This change allows single-byte access to port 501 and also uses the value
written to construct an exit code.

Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
