Continuing the discussion from Breaking changes announcement for unstable:
Hi @afics,
As outlined in nixos/libvirtd: Make all OVMF images from QEMU package available by bdd · Pull Request #421549 · NixOS/nixpkgs · GitHub , support for custom OVMF builds may be reintroduced. It requires extending the current packaging for OVMF to include the descriptors and modify the libvirt module such that it copies those descriptors to the proper location. The comment also has some workarounds that can be used for now.