Describe the bug
A clear and concise description of what the bug is.

To Reproduce
Steps to reproduce the behavior:
1.

(import <nixpkgs/nixos> {
  configuration = ({ ... }: {
    users.users.root.password = "";

    services.getty.autologinUser = "root";

    virtualisation.qemu.options = [
      "-virtfs local,path=/home,security_model=none,mount_tag=testmount"
    ];

    fileSystems."/testmount" =
      { device = "testmount";
        fsType = "9p";
        options = [ "trans=virtio" "version=9p2000.L" ];
        neededForBoot = true;
      };
  });
}).vm

2. nix-build
3. result/bin/run-nixos-vm
4. ls /testmount # will fail

Expected behavior
/testmount should exist and be mounted

Additional context
/testmount is not even mentioned in /etc/fstab

possibly the mkVMOverride on the whole fileSystems attrset is causing this

Notify maintainers

Metadata

• system: "x86_64-linux"
• host os: Linux 5.4.96, NixOS, 21.03pre-git (Okapi)
• multi-user?: yes
• sandbox: yes
• version: nix-env (Nix) 2.4pre20201205_a5d85d0
• channels(root): "nixos-21.03pre268835.8c8731330b5, nixpkgs, solaros"
• channels(maciej): "nixpkgs, solaros"
• nixpkgs: /home/maciej/.nix-defexpr/channels/nixpkgs

Maintainer information:

# a list of nixpkgs attributes affected by the problem
attribute:
# a list of nixos modules affected by the problem
module: