libxl: ocaml: provide defaults for libxl types

libxl: ocaml: provide defaults for libxl types

Libxl functions such as libxl_domain_create_new take large structs
of configuration parameters. Often, we would like to use the default
values for many of these parameters.

The struct and keyed-union types in libxl have init functions, which
fill in the defaults for a given type. This commit provides an OCaml
interface to obtain records of defaults by calling the relevant init
function.

These default records can be used as a base to construct your own
records, and to selectively override parameters where needed.

For example, a Domain_create_info record can now be created as follows:

  Xenlight.Domain_create_info.({ default ctx () with
    ty = Xenlight.DOMAIN_TYPE_PV;
    name = Some vm_name;
    uuid = vm_uuid;
  })

For types with KeyedUnion fields, such as Domain_build_info, a record
with defaults is obtained by specifying the type key:

  Xenlight.Domain_build_info.default ctx ~ty:Xenlight.DOMAIN_TYPE_HVM ()

Signed-off-by: Rob Hoes <rob.hoes@citrix.com>
Acked-by: David Scott <dave.scott@eu.citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>