Fix Nix database in generated images

This prevents seeing lots of warnings about missing hashes/sizes in the
database when running "nix-store --verify --check-contents" for the
first time.

nixos/lib/make-disk-image.nix

@@ -79,6 +79,10 @@ pkgs.vmTools.runInLinuxVM (
       printRegistration=1 perl ${pkgs.pathsFromGraph} /tmp/xchg/closure | \
           chroot /mnt ${config.nix.package}/bin/nix-store --load-db --option build-users-group ""
 
+      # Add missing size/hash fields to the database. FIXME:
+      # exportReferencesGraph should provide these directly.
+      chroot /mnt ${config.nix.package}/bin/nix-store --verify --check-contents
+
       # Create the system profile to allow nixos-rebuild to work.
       chroot /mnt ${config.nix.package}/bin/nix-env --option build-users-group "" \
           -p /nix/var/nix/profiles/system --set ${config.system.build.toplevel}