Extend the Template Haskell loadFn model

Currently, we only allow field-by-field de-serialisation. Since we
have cases where information about how to un-serialise a field is
split across two JSON fields (e.g. disk type and disk logical_id,
hypervisor and hvparams, etc.), we need to pass the entire object to
custom read functions.

Furthermore, since we will have to generate two actual fields from the
single in-memory field, we need to extend the custom save function so
that they can generate additional fields beyond the "main" field value
they currently generate.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Agata Murawska <agatamurawska@google.com>