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: NAnthony Liguori <aliguori@us.ibm.com>

hw/pc.c

@@ -549,8 +549,7 @@ static void bochs_bios_write(void *opaque, uint32_t addr, uint32_t val)
         /* LGPL'ed VGA BIOS messages */
     case 0x501:
     case 0x502:
-        fprintf(stderr, "VGA BIOS panic, line %d\n", val);
-        exit(1);
+        exit((val << 1) | 1);
     case 0x500:
     case 0x503:
 #ifdef DEBUG_BIOS
@@ -591,6 +590,7 @@ static void *bochs_bios_init(void)
     register_ioport_write(0x403, 1, 1, bochs_bios_write, NULL);
     register_ioport_write(0x8900, 1, 1, bochs_bios_write, NULL);
 
+    register_ioport_write(0x501, 1, 1, bochs_bios_write, NULL);
     register_ioport_write(0x501, 1, 2, bochs_bios_write, NULL);
     register_ioport_write(0x502, 1, 2, bochs_bios_write, NULL);
     register_ioport_write(0x500, 1, 1, bochs_bios_write, NULL);