docs(nixos): create vm.nix for explore microvm.nix

in nixosConfigurations/vm.nix

nix/nixosConfigurations/vm.nix

@@ -0,0 +1,69 @@
+# This is example how to use github:astro/microvm.nix as host (NixOS) as a VM Server
+# and have aarch64 VMs running on it.
+#
+# Limitations:
+# * The VM (`aarch64`) cannot run correctly, because host (`microvm-host`) is running on qemu `linux-builder`
+
+{
+  inputs,
+  ezModules,
+  ...
+}:
+
+{
+  imports = [
+    "${inputs.nixpkgs}/nixos/modules/profiles/nix-builder-vm.nix"
+    inputs.microvm.nixosModules.host
+    (
+      { pkgs, lib, ... }:
+      {
+        microvm = rec {
+          autostart = lib.attrNames vms;
+          vms = {
+            aarch64 = {
+              inherit pkgs;
+              config = {
+                microvm = {
+                  volumes = [
+                    {
+                      mountPoint = "/var";
+                      image = "var.img";
+                      size = 256;
+                    }
+                  ];
+                  shares = [
+                    {
+                      proto = "9p";
+                      tag = "ro-store";
+                      source = "/nix/store";
+                      mountPoint = "/nix/.ro-store";
+                    }
+                  ];
+                  hypervisor = "qemu";
+                  socket = "control.socket";
+                };
+                environment.systemPackages = with pkgs; [
+                  cowsay
+                  htop
+                ];
+              };
+            };
+          };
+        };
+      }
+    )
+
+    # --- my configurations
+    ezModules.nix
+    {
+      # --- see: nix/nixosModules/nix.nix
+      nix-settings.enable = true;
+
+      nixpkgs.hostPlatform = "aarch64-linux";
+      system.stateVersion = "24.05";
+      networking.hostName = "microvm-host";
+      users.users.root.password = "";
+      users.users.root.openssh.authorizedKeys.keys = inputs.self.users.r17.keys;
+    }
+  ];
+}