Fix the EC2 test

We now generate a qcow2 image to prevent hitting Hydra's output size
limit. Also updated /root/user-data -> /etc/ec2-metadata/user-data.

http://hydra.nixos.org/build/33843133

nixos/lib/make-disk-image.nix

@@ -23,6 +23,8 @@
   postVM ? ""
 
 , name ? "nixos-disk-image"
+
+, format ? "raw"
 }:
 
 with lib;
@@ -32,8 +34,8 @@ pkgs.vmTools.runInLinuxVM (
     { preVM =
         ''
           mkdir $out
-          diskImage=$out/nixos.img
-          ${pkgs.vmTools.qemu}/bin/qemu-img create -f raw $diskImage "${toString diskSize}M"
+          diskImage=$out/nixos.${if format == "qcow2" then "qcow2" else "img"}
+          ${pkgs.vmTools.qemu}/bin/qemu-img create -f ${format} $diskImage "${toString diskSize}M"
           mv closure xchg/
         '';
       buildInputs = [ pkgs.utillinux pkgs.perl pkgs.e2fsprogs pkgs.parted ];