This is a complex field, so we have to do a lot of manual work for now.

The complexity arises from the fact that the contents of the field,
and the way to parse it, depends on the disk type field, so we don't
have a single, static way of parsing it. Hence we needed the
extensions to the Template Haskell code.

Since we now can both load and save the disk type, we can remove the
in-memory (duplicate) disk type from the disk objects, relying only on
the logical ID to hold the type information.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: René Nussbaumer <rn@google.com>