pckbd: set bits 2-3-6-7 of the output port by default

OSes typically write 0xdd/0xdf to turn the A20 line off and on.  This
has bits 2-3-6-7 on, so that the output port subsection is migrated.
Change the reset value and migration default to include those four
bits, thus avoiding that the subsection is migrated.

This strictly speaking changes guest ABI, but the long time during which
we have not migrated the value means that the guests really do not care
much; so the change is for all machine types.
Reported-by: NIgor Mammedov <imammedo@redhat.com>
Cc: qemu-stable@nongnu.org
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

hw/input/pckbd.c

@@ -101,6 +101,12 @@
 #define KBD_OUT_OBF             0x10    /* Keyboard output buffer full */
 #define KBD_OUT_MOUSE_OBF       0x20    /* Mouse output buffer full */
 
+/* OSes typically write 0xdd/0xdf to turn the A20 line off and on.
+ * We make the default value of the outport include these four bits,
+ * so that the subsection is rarely necessary.
+ */
+#define KBD_OUT_ONES            0xcc
+
 /* Mouse Commands */
 #define AUX_SET_SCALE11		0xE6	/* Set 1:1 scaling */
 #define AUX_SET_SCALE21		0xE7	/* Set 2:1 scaling */
@@ -367,13 +373,13 @@ static void kbd_reset(void *opaque)
 
     s->mode = KBD_MODE_KBD_INT | KBD_MODE_MOUSE_INT;
     s->status = KBD_STAT_CMD | KBD_STAT_UNLOCKED;
-    s->outport = KBD_OUT_RESET | KBD_OUT_A20;
+    s->outport = KBD_OUT_RESET | KBD_OUT_A20 | KBD_OUT_ONES;
     s->outport_present = false;
 }
 
 static uint8_t kbd_outport_default(KBDState *s)
 {
-    return KBD_OUT_RESET | KBD_OUT_A20
+    return KBD_OUT_RESET | KBD_OUT_A20 | KBD_OUT_ONES
            | (s->status & KBD_STAT_OBF ? KBD_OUT_OBF : 0)
            | (s->status & KBD_STAT_MOUSE_OBF ? KBD_OUT_MOUSE_OBF : 0);
 }