nixos/qemu-vm: Don't assume boot drive is always vdb

nixos/modules/virtualisation/qemu-vm.nix

@@ -46,6 +46,13 @@ let
         description = "Extra options passed to device flag.";
       };
 
+      name = mkOption {
+        type = types.nullOr types.str;
+        default = null;
+        description =
+          "A name for the drive. Must be unique in the drives list. Not passed to qemu.";
+      };
+
     };
 
   };
@@ -84,6 +91,14 @@ let
     else
       "/dev/vd${letter}";
 
+  lookupDriveDeviceName = driveName: driveList:
+    (findSingle (drive: drive.name == driveName)
+      (throw "Drive ${driveName} not found")
+      (throw "Multiple drives named ${driveName}") driveList).device;
+
+  addDeviceNames =
+    imap1 (idx: drive: drive // { device = driveDeviceName idx; });
+
   # Shell script to start the VM.
   startVM =
     ''
@@ -406,6 +421,7 @@ in
         mkOption {
           type = types.listOf (types.submodule driveOpts);
           description = "Drives passed to qemu.";
+          apply = addDeviceNames;
         };
 
       diskInterface =
@@ -552,12 +568,14 @@ in
 
     virtualisation.qemu.drives = mkMerge [
       [{
+        name = "root";
         file = "$NIX_DISK_IMAGE";
         driveExtraOpts.cache = "writeback";
         driveExtraOpts.werror = "report";
       }]
       (mkIf cfg.useBootLoader [
         {
+          name = "boot";
           file = "$TMPDIR/disk.img";
           driveExtraOpts.media = "disk";
           deviceExtraOpts.bootindex = "1";
@@ -610,7 +628,7 @@ in
           };
       } // optionalAttrs cfg.useBootLoader
       { "/boot" =
-          { device = "${driveDeviceName 1}2";
+          { device = "${lookupDriveDeviceName "boot" cfg.qemu.drives}2";
             fsType = "vfat";
             options = [ "ro" ];
             noCheck = true; # fsck fails on a r/o filesystem