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/maintainers/scripts/ec2/amazon-image.nix

@@ -10,9 +10,11 @@ with lib;
     ];
 
   system.build.amazonImage = import ../../../lib/make-disk-image.nix {
-    inherit pkgs lib config;
+    inherit lib config;
+    pkgs = import ../../../.. { inherit (pkgs) system; }; # ensure we use the regular qemu-kvm package
     partitioned = config.ec2.hvm;
     diskSize = if config.ec2.hvm then 2048 else 8192;
+    format = "qcow2";
     configFile = pkgs.writeText "configuration.nix"
       ''
         {