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.