{qemu,vz}-vm: load registration from lower store

Otherwise, if gc later decides to remove registration derivation from
its store, consequent reboots will point to a missing registration file.

(As of this patch, this is not fatal because registration service
failure doesn't fail builder boot; a later patch in the series makes
registration service required, making sure we never boot with db that is
not populated.)

nixos/modules/virtualisation/qemu-vm.nix

@@ -351,6 +351,11 @@ let
   '';
 
   regInfo = hostPkgs.closureInfo { rootPaths = config.virtualisation.additionalPaths; };
+  regInfoPath =
+    if cfg.useNixStoreImage || cfg.mountHostNixStore then
+      "/nix/.ro-store/${baseNameOf regInfo}/registration"
+    else
+      "${regInfo}/registration";
 
   # Use well-defined and persistent filesystem labels to identify block devices.
   rootFilesystemLabel = "nixos";
@@ -1314,7 +1319,7 @@ in
         mkIf cfg.directBoot.enable [
           "-kernel \${NIXPKGS_QEMU_KERNEL_${sanitizeShellIdent config.system.name}:-${config.system.build.toplevel}/kernel}"
           "-initrd ${cfg.directBoot.initrd}"
-          ''-append "$(cat ${config.system.build.toplevel}/kernel-params) init=${config.system.build.toplevel}/init regInfo=${regInfo}/registration ${consoles} $QEMU_KERNEL_PARAMS"''
+          ''-append "$(cat ${config.system.build.toplevel}/kernel-params) init=${config.system.build.toplevel}/init regInfo=${regInfoPath} ${consoles} $QEMU_KERNEL_PARAMS"''
         ]
       )
       (mkIf cfg.useEFIBoot [

nixos/modules/virtualisation/vz-vm.nix

@@ -14,6 +14,7 @@ let
   hostPkgs = cfg.host.pkgs;
 
   regInfo = hostPkgs.closureInfo { rootPaths = cfg.additionalPaths; };
+  regInfoPath = "/nix/.ro-store/${baseNameOf regInfo}/registration";
 
   # Host-built erofs store image, named by closure hash so only a changed guest rebuilds it.
   # A derivation is not an option: it would need the Linux builder this VM provides.
@@ -31,7 +32,7 @@ let
     # Virtualization.framework offers a virtio console
     "console=hvc0"
     "init=${toplevel}/init"
-    "regInfo=${regInfo}/registration"
+    "regInfo=${regInfoPath}"
   ]
   ++ config.boot.kernelParams;