Add machineConfigModule parameter support to all distro modules

[Mic92]

This PR adds support for the machineConfigModule parameter to all distro modules (ubuntu, debian, fedora), allowing users to customize VM configuration such as memory size, CPU count, and other virtualisation settings.

Changes

All distro modules (ubuntu, debian, fedora):

• Added machineConfigModule parameter to makeVmTestForImage function
• Parameter defaults to generic.defaultMachineConfigModule for backward compatibility
• Passes the parameter through to generic.makeVmTest

Debian module specifically:

• Fixed missing extraPathsToRegister parameter in makeVmTestForImage function
• This ensures consistency with ubuntu and fedora modules

Usage

Users can now customize VM configuration like this:

nix-vm-test.lib.x86_64-linux.ubuntu."22_04" {
  testScript = "...";
  sharedDirs = {};
  machineConfigModule = { ... }: {
    nodes.vm.virtualisation.memorySize = 2048;  # 2GB RAM
    nodes.vm.virtualisation.cpus = 2;           # 2 CPU cores
  };
}

Backward Compatibility

All changes are backward compatible - existing code will continue to work without modification.

[picnoir]

Hmm, this API was not originally meant to be public. Doing this, we're exposing a few footguns (the vlan and rootImage part mostly). We probably should clean this up/filter those, but it's fine to do that in a subsequent PR.

I'm fine to merge this if you add a brief description of this new API to the reference documentation?

nix-vm-test/doc/reference.md

Line 122 in 2440f3a

#### Inputs

(at least for memorySize and cpus).

[picnoir]

On second thought, a proper way to do this would be to add memorySize and cpus, the only bits that really matter, to the makeVmTestForImage APIs. There's one call site per distribution. We could probably merge all that.

[Mic92]

When I was looking at the code I also wondering why nix-vm-test is not just a module for the nixos vm test module system? This is how we implemented our clan vm tests and nixos container. Is this something you considered?