spapr: Migrate ail-mode-3 spapr cap

This cap did not add the migration code when it was introduced. This
results in migration failure when changing the default using the
command line.

Cc: qemu-stable@nongnu.org
Fixes: ccc5a4c5e1 ("spapr: Add SPAPR_CAP_AIL_MODE_3 for AIL mode 3 support for H_SET_MODE hcall")
Reviewed-by: Harsh Prateek Bora <harshpb@linux.ibm.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>

hw/ppc/spapr.c

@@ -2195,6 +2195,7 @@ static const VMStateDescription vmstate_spapr = {
         &vmstate_spapr_cap_fwnmi,
         &vmstate_spapr_fwnmi,
         &vmstate_spapr_cap_rpt_invalidate,
+        &vmstate_spapr_cap_ail_mode_3,
         &vmstate_spapr_cap_nested_papr,
         NULL
     }