spapr: Migrate ail-mode-3 spapr cap

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: ccc5a4c5e10 ("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>