qemu_boot_set() can't fail in restore_boot_order(),
then simply assert it doesn't fail, by passing
&error_abort if boot_set_handler set.

Suggested-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Gonglei <arei.gonglei@huawei.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>