VMs: persist state folders on host

Done:
- move vm inspect attrs from system.clan.vm.config to clanCore.vm.inspect. This gives us proper name and type checking. everything in `system` is basically freeform, so the previous option definitions were never enforced
- when running VMs, mount state directory from ~/.config/clan/vmstate/{...} from the host to /var/vmstate inside the vm
- create bind mount inside the VM from /var/vmstate/{folder} to / for all folders defined in clanCore.state.<name>.folders

TODOs:
- make sure directories in ~/.config/clan/vmstate never collide (include hash of clan-url, etc.)
- port impure test to python

nixosModules/clanCore/outputs.nix

@@ -45,12 +45,6 @@
           '';
           default = "${pkgs.coreutils}/bin/true";
         };
-        vm.config = lib.mkOption {
-          type = lib.types.attrs;
-          description = ''
-            the vm config
-          '';
-        };
         vm.create = lib.mkOption {
           type = lib.types.path;
           description = ''

nixosModules/clanCore/vm.nix

@@ -1,19 +1,59 @@
 { lib, config, pkgs, options, extendModules, modulesPath, ... }:
 let
-  vmConfig = extendModules {
-    modules = [
+  # Generates a fileSystems entry for bind mounting a given state folder path
+  # It binds directories from /var/clanstate/{some-path} to /{some-path}.
+  # As a result, all state paths will be persisted across reboots, because
+  #   the state folder is mounted from the host system.
+  mkBindMount = path: {
+    name = path;
+    value = {
+      device = "/var/clanstate/${path}";
+      options = [ "bind" ];
+    };
+  };
+
+  # Flatten the list of state folders into a single list
+  stateFolders = lib.flatten (
+    lib.mapAttrsToList
+      (_item: attrs: attrs.folders)
+      config.clanCore.state
+  );
+
+  # A module setting up bind mounts for all state folders
+  stateMounts = {
+    virtualisation.fileSystems =
+      lib.listToAttrs
+        (map mkBindMount stateFolders);
+  };
+
+  vmModule = {
+    imports = [
       (modulesPath + "/virtualisation/qemu-vm.nix")
       ./serial.nix
-      {
-        virtualisation.fileSystems.${config.clanCore.secretsUploadDirectory} = lib.mkForce {
-          device = "secrets";
-          fsType = "9p";
-          neededForBoot = true;
-          options = [ "trans=virtio" "version=9p2000.L" "cache=loose" ];
-        };
-        boot.initrd.systemd.enable = true;
-      }
+      stateMounts
     ];
+    virtualisation.fileSystems = {
+      ${config.clanCore.secretsUploadDirectory} = lib.mkForce {
+        device = "secrets";
+        fsType = "9p";
+        neededForBoot = true;
+        options = [ "trans=virtio" "version=9p2000.L" "cache=loose" ];
+      };
+      "/var/clanstate" = {
+        device = "state";
+        fsType = "9p";
+        options = [ "trans=virtio" "version=9p2000.L" "cache=loose" ];
+      };
+    };
+    boot.initrd.systemd.enable = true;
+  };
+
+  # We cannot simply merge the VM config into the current system config, because
+  # it is not necessarily a VM.
+  # Instead we use extendModules to create a second instance of the current
+  # system configuration, and then merge the VM config into that.
+  vmConfig = extendModules {
+    modules = [ vmModule stateMounts ];
   };
 in
 {
@@ -47,17 +87,54 @@ in
         '';
       };
     };
+    # All important VM config variables needed by the vm runner
+    # this is really just a remapping of values defined elsewhere
+    # and therefore not intended to be set by the user
+    clanCore.vm.inspect = {
+      clan_name = lib.mkOption {
+        type = lib.types.str;
+        internal = true;
+        readOnly = true;
+        description = ''
+          the name of the clan
+        '';
+      };
+      memory_size = lib.mkOption {
+        type = lib.types.int;
+        internal = true;
+        readOnly = true;
+        description = ''
+          the amount of memory to allocate to the vm
+        '';
+      };
+      cores = lib.mkOption {
+        type = lib.types.int;
+        internal = true;
+        readOnly = true;
+        description = ''
+          the number of cores to allocate to the vm
+        '';
+      };
+      graphics = lib.mkOption {
+        type = lib.types.bool;
+        internal = true;
+        readOnly = true;
+        description = ''
+          whether to enable graphics for the vm
+        '';
+      };
+    };
   };
 
   config = {
+    # for clan vm inspect
+    clanCore.vm.inspect = {
+      clan_name = config.clanCore.clanName;
+      memory_size = config.clan.virtualisation.memorySize;
+      inherit (config.clan.virtualisation) cores graphics;
+    };
+    # for clan vm create
     system.clan.vm = {
-      # for clan vm inspect
-      config = {
-        clan_name = config.clanCore.clanName;
-        memory_size = config.clan.virtualisation.memorySize;
-        inherit (config.clan.virtualisation) cores graphics;
-      };
-      # for clan vm create
       create = pkgs.writeText "vm.json" (builtins.toJSON {
         initrd = "${vmConfig.config.system.build.initialRamdisk}/${vmConfig.config.system.boot.loader.initrdFile}";
         toplevel = vmConfig.config.system.build.toplevel;