runInLinuxVM: apply Qemu's vCPU limit

[pogobanane]

Problem:

pkgs.vmTools.runInLinuxVM is used, e.g. by make-disk-image.nix and some tests. Its starts a Qemu instance but may fail on systems with >= 256 CPU cores because Qemu for now only supports 255 cores.

Solution:

Limit number of virtual CPUs created by Qemu to 255.

Tested on a 256 core system.

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: nix-build ./pkgs/build-support/vm/test.nix
• 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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