qemu: Enable smbd support by default.

[nh2]

Motivation for this change

Shared folders with Windows guests are an important feature for both desktop and server environments.

#41615 fixed the support, this enables it by default; it took me hours to figure out why this feature, which is expected to be working according to the qemu manual, didn't work out of the box on NixOS.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

CC @eqyiel @xeji from #41615

Slightly related, for my memory: I also just submitted this fix to qemu upstream, which is required to make it work well on current Samba: nh2/qemu@de30898

[bb2020]

#94370

[nh2]

@bb2020 Ah OK, that's good. What if I use the kvm package (e.g. nix-shell -p kvm), doesn't that pull in the non-samba version?

Thanks!

[bb2020]

Yes, that pulls non-samba version. Only qemu_full has samba and ceph support built-in.

[nh2]

Yes, that pulls non-samba version. Only qemu_full has samba and ceph support built-in.

@bb2020 is it safe to use that instead of the kvm package, or am I at risk of not having it hardware-accelerated then?

[bb2020]

It is the same. kvm is just an alias for qemu-host-cpu-only. Hardware acceleration is actually determined by
-machine accel=kvm flag that you pass to qemu.