pc: port 92 reset requires a low->high transition

The PIIX datasheet says that "before another INIT pulse can be
generated via [port 92h], [bit 0] must be written back to a
zero.

This bug is masked right now because a full reset will clear the
value of port 92h.  But once we implement soft reset correctly,
the next attempt to enable the A20 line by setting bit 1 (and
leaving the others untouched) will cause another reset.
Reviewed-by: NAnthony Liguori <aliguori@us.ibm.com>
Reviewed-by: NMichael S. Tsirkin <mst@redhat.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

hw/i386/pc.c

@@ -471,11 +471,12 @@ static void port92_write(void *opaque, hwaddr addr, uint64_t val,
                          unsigned size)
 {
     Port92State *s = opaque;
+    int oldval = s->outport;
 
     DPRINTF("port92: write 0x%02x\n", val);
     s->outport = val;
     qemu_set_irq(*s->a20_out, (val >> 1) & 1);
-    if (val & 1) {
+    if ((val & 1) && !(oldval & 1)) {
         qemu_system_reset_request();
     }
 }