migration: Add Error** argument to add_bitmaps_to_list()

migration: Add Error** argument to add_bitmaps_to_list()

This allows to report more precise errors in the migration handler
dirty_bitmap_save_setup().

Suggested-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
Signed-off-by: Cédric Le Goater <clg@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
Link: https://lore.kernel.org/r/20240329105627.311227-1-clg@redhat.com
Signed-off-by: Peter Xu <peterx@redhat.com>