Fix virtualbox deployment for 19.09

Without this, starting the virtualbox guest service fails during machine activation.
This is due to an incompatible NixOS machine base image.
Fix this by using an updated version of nixops.

pkgs/nixops/default.nix

@@ -0,0 +1,53 @@
+# An updated version of nixops that's compatible with machines running NixOS 19.09.
+# 19.09 demands a suitable base image (defined in nixops-vbox/nix/virtualbox.nix) to
+# start the virtualbox guest service during system activation.
+
+{ stdenv, nixops, runCommand, fetchFromGitHub }:
+
+let
+  pluginData = {
+    aws = {
+      owner = "nixos";
+      repo = "nixops-aws";
+      rev= "v1.0.0";
+      sha256 = "1if6spscsgd6ckivgvbqza5fvvn5hbafi1n8q0fw98s3xpz2hjfm";
+    };
+    hetzner = {
+      owner = "nixos";
+      repo = "nixops-hetzner";
+      rev = "v1.0.0";
+      sha256 = "0cxfjpk2daczv3m7q5bsgfvd30qgmm1y7dnvz6nd7s7l7l0gsvas";
+    };
+    vbox = {
+      owner = "nix-community";
+      repo = "nixops-vbox";
+      rev = "bff6054ce9e7f5f9aa830617577f1a511a461063";
+      sha256 = "0j0lbi8rqmw17ji367zh94lvlb062iiyavl4l7m851v40wqr8a5i";
+    };
+    libvirtd = {
+      owner = "nix-community";
+      repo = "nixops-libvirtd";
+      rev = "v1.0.0";
+      sha256 = "0g2ag4mhgrxws3h4q8cvfh4ks1chgpjm018ayqd48lagyvi32l8m";
+    };
+  };
+
+  origSrc = fetchFromGitHub {
+    owner = "NixOS";
+    repo = "nixops";
+    rev = "2434bf26e0bba49441041ffce36dc324f049bc00";
+    sha256 = "0ag05pjwwqdw8in49hr8m8bdg31xsgqs1cawcqyh6a5lsys7f6zg";
+  };
+
+  src = runCommand "src" {} ''
+    cp --no-preserve=mode -r ${origSrc} $out
+    cd $out
+    patch -p1 < ${./release.nix.patch}
+  '';
+
+  nixopsRelease = import "${src}/release.nix" {
+    inherit pluginData;
+    p = (p: with p; [ aws hetzner vbox libvirtd ]);
+  };
+in
+nixopsRelease.build.${builtins.currentSystem}