nixos/make-disk-image.nix: Support EFI images

- Add a new parameter `imageType` that can specify either "efi" or
  "legacy" (the default which should see no change in behaviour by
  this patch).

- EFI images get a GPT partition table (instead of msdos) with a
  mandatory ESP partition (so we add an assert that `partitioned`
  is true).

- Use the partx tool from util-linux to determine exact start + size
  of the root partition. This is required because GPT stores a secondary
  partition table at the end of the disk, so we can't just have
  mkfs.ext4 create the filesystem until the end of the disk.

- (Unrelated to any EFI changes) Since we're depending on the
  `-E offset=X` option to mkfs which is only supported by e2fsprogs,
  disallow any attempts of creating partitioned disk images where
  the root filesystem is not ext4.

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

@@ -46,7 +46,7 @@ in {
     inherit lib config;
     inherit (cfg) contents format name;
     pkgs = import ../../../.. { inherit (pkgs) system; }; # ensure we use the regular qemu-kvm package
-    partitioned = config.ec2.hvm;
+    partitionTableType = if config.ec2.hvm then "legacy" else "none";
     diskSize = cfg.sizeMB;
     configFile = pkgs.writeText "configuration.nix"
       ''