xen: fix seabios path

[CertainLach]

Description of changes

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandboxing enabled in nix.conf? (See Nix manual)
  • sandbox = relaxed
  • sandbox = true
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 24.11 Release Notes (or backporting 23.11 and 24.05 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
• Fits CONTRIBUTING.md.

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[SigmaSquadron]

Ideally we'd have something similar to OVMF.firmware. @AndersonTorres as the SeaBIOS maintainer, would you be opposed to a passthru.firmware attribute that pointed to ${finalAttrs.finalPackage}/share/seabios/${firmware} where firmware is Csm16.bin by default?

[SigmaSquadron]

Also, I'm assuming #341429 will need to override seabios to point to qubes-seabios? (which will have the same passthru.firmware but that points to the Qubes-specific executable.)

[CertainLach]

Yes, that's right. qubes-seabios is packaged in qubes-packages PR, wanted firmware filename is "bios-256k.bin"

[SigmaSquadron]

Yes, that's right. qubes-seabios is packaged in qubes-packages PR, wanted firmware filename is "bios-256k.bin"

Add something like this to qubes-seabios:

passthru.firmware = "${finalAttrs.finalPackage}/share/seabios/bios-256k.bin";

and once we get the firmware attr in the main seabios package, it'll Just Work™ and is cleaner than another override.

[AndersonTorres]

To me, no problems. I am working on this:

#342692