qemu_xen: unbreak, add missing deps

[pbsds]

fixes https://hydra.nixos.org/build/295921651
zhf #403336

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/)
• 25.05 Release Notes (or backporting 24.11 and 25.05 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]

GitHub confuses and frightens me sometimes. Sorry for the accidental deletion; here's the original comment:

I've attempted this through an alternative means here: #403479.

I also wanted to add in a review comment the following:

I'd hate for qemu_xen to be the only one with fixes regarding libepoxy, especially since Xen doesn't care about X11 or GL at all!

If propagating the inputs isn't the way to go, then I'd rather have qemu_xen built with minimal = true, but I'm unsure if we'd be leaving something out that Xen depends on.