flake: VM: add additional disk with empty ext4 FS

flake.nix

@@ -113,10 +113,16 @@
           "black --check ${self.outPath} > $out";
         default =
           pkgs.testers.runNixOSTest {
-            imports = [{
             name = "default";
             nodes.machine = { lib, pkgs, ... }: {
-                imports = [{
+              # additional disk of size 1024 MiB with empty ext4 FS
+              virtualisation.emptyDiskImages = [ 1024 ];
+              virtualisation.fileSystems."/volumes/vdb" = {
+                autoFormat = true;
+                device = "/dev/vdb"; # this name is chosen by QEMU, not here
+                fsType = "ext4";
+                noCheck = true;
+              };
               boot.consoleLogLevel = lib.mkForce 3;
               documentation.enable = false;
               services.journald.extraConfig = lib.mkForce "";
@@ -135,7 +141,6 @@
               environment.variables.TEST_MODE = "true";
               systemd.tmpfiles.settings.src.${vmtest-src-dir}.L.argument =
                 self.outPath;
-                }];
             };
             testScript = ''
               start_all()
@@ -144,7 +149,6 @@
               machine.copy_from_vm("coverage.xml", ".")
               machine.succeed("coverage report >&2")
             '';
-            }];
           };
       };
     };