Comment 37 for bug 1882671

  <loader readonly='yes' type='pflash'>/usr/share/OVMF/OVMF_CODE.fd</loader>
  <nvram template='/usr/share/OVMF/OVMF_VARS.fd'>/var/lib/uvtool/libvirt/images/bionic.VARS.fd</nvram>

makes it run eif ovmf EFI like
-drive file=/usr/share/OVMF/OVMF_CODE.fd,if=pflash,format=raw,unit=0,readonly=on -drive file=/var/lib/uvtool/libvirt/images/bionic.VARS.fd,if=pflash,format=raw,unit=1

Still mgriates well from Bionic -> Focal and Back
B
$ virsh migrate --unsafe --live bionic qemu+ssh://10.225.185.44/system
F
$ virsh migrate --unsafe --live bionic qemu+ssh://10.225.185.35/system
B
$ virsh console bionic
Connected to domain bionic
Escape character is ^]
Ubuntu 18.04.4 LTS bionic ttyS0
bionic login:

So that also works well.
The only possible option missing to be tested is the real https boot via Focals OVMF (which didn't have https enabled), but unless someone comes back explaining that there is an odd combination that could have got that going we should be good.

Let is sit a bit more time in proposed to be sure and wait for feedback.
Then we can declare it fully verified.