While this thread has been solved, I’ll write in actual words how to fix the issue for those who are in a rush and don’t wish to watch a video: all you need to do is set boot.priority for the ISO-image you want to boot, like e.g. lxc config device set myvirtualmachine iso boot.priority=10 – POOF, UEFI will boot from the ISO-image by default.
You asked how to get EFI to boot from the correct device and you do that with boot.priority. You still need to do the other extra steps as well, if you want to boot anything based on FreeBSD.