Depend on edk2-ovmf-xen instead of bundling OVMF

[DemiMarie]

This fixes two bugs:

1. The bundled OVMF didn't support Xen, so UEFI guests failed to boot.
2. The bundled OVMF didn't include the license text alongside it.

Fixes: QubesOS/qubes-issues#8625

[marmarek]

Looks good, but I'd like to make a test for that first. If you want to help, take a look at simple VM startup tests in core-admin, and add one that checks if UEFI VM boots too (and test if this PR really fixes it).

[DemiMarie]

When I tested (an equivalent to) this myself, it was enough to get the firmware loading, but I was dumped into a UEFI shell because /boot/efi could not be mounted in the firmware. I would prefer to merge this change first, and then fix the problems inside the guest that prevent booting past the UEFI shell.

[marmarek]

I'm going to insist on a test in core-admin, but if you don't want to, I can add it too. It doesn't need to test full qubes-template-based qube booting with UEFI. In fact, I believe that's very minor use case for this feature. It can be something minimal - maybe even just grub configured to print something to the console? (see the cdrom boot test, that one uses syslinux, but you get the idea).
This would for example show that this PR itself results in a VM that still fails to boot at all in R4.3 (with updated libvirt).

[DemiMarie]

I’ll add the test. I just thought you wanted me to test that a standard Qubes VM booted with UEFI firmware (which it doesn’t).