xen_4_8: init at 4.8.1

This commit adds the xen_4_8 package to be used instead of
xen (currently at 4.5.5):
 * Add packages xen_4_8, xen_4_8-slim and xen_4_8-light
 * Add packages qemu_xen_4_8 and qemu_xen_4_8-light to be used
   with xen_4_8-slim and xen_4_8-light respectively.
 * Add systemd to buildInputs of xen (it is required by oxenstored)
 * Adapt xen service to work with the new version of xen
 * Use xen-init-dom0 to initlilise dom0 in xen-store
 * Currently, the virtualisation.xen.stored option is ignored
   if xen 4.8 is used

pkgs/applications/virtualization/xen/packages.nix

@@ -57,6 +57,56 @@ rec {
     };
   };
 
+  xen_4_8-vanilla = callPackage ./4.8.nix {
+    # At the very least included seabios and etherboot need gcc49,
+    # so we have to build all of it with gcc49.
+    stdenv = overrideCC stdenv gcc49;
+
+    meta = {
+      description = "vanilla";
+      longDescription = ''
+        Vanilla version of Xen. Uses forks of Qemu and Seabios bundled
+        with Xen. This gives vanilla experince, but wastes space and
+        build time: typical NixOS setup that runs lots of VMs will
+        build three different versions of Qemu when using this (two
+        forks and upstream).
+      '';
+    };
+  };
+
+  xen_4_8-slim = xen_4_8-vanilla.override {
+    withInternalQemu = false;
+    withInternalTraditionalQemu = true;
+    withInternalSeabios = false;
+    withSeabios = true;
+
+    meta = {
+      description = "slim";
+      longDescription = ''
+        Slimmed-down version of Xen that reuses nixpkgs packages as
+        much as possible. Different parts may get out of sync, but
+        this builds faster and uses less space than vanilla. Use with
+        `qemu_xen` from nixpkgs.
+      '';
+    };
+  };
+
+  xen_4_8-light = xen_4_8-vanilla.override {
+    withInternalQemu = false;
+    withInternalTraditionalQemu = false;
+    withInternalSeabios = false;
+    withSeabios = true;
+
+    meta = {
+      description = "light";
+      longDescription = ''
+        Slimmed-down version of Xen without `qemu-traditional` (you
+        don't need it if you don't know what it is). Use with
+        `qemu_xen-light` from nixpkgs.
+      '';
+    };
+  };
+
   xen-vanilla = xen_4_5-vanilla;
   xen-slim = xen_4_5-slim;
   xen-light = xen_4_5-light;