tools: ocaml: lay ground work for auto generating xl datatypes.

tools: ocaml: lay ground work for auto generating xl datatypes.

Doesn't actually generate anything yet but puts all the moving parts
into place. In particular sets up the
xl.ml.in+_libxl_types.ml.in->xl.ml transformation using sed. This
appears to be the only/best way to do this for ocaml due to the lack
of a preprocessor and/or an include mechanism which has an inmpact on
namespacing.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Ian Jackson <ian.jackson.citrix.com>
Committed-by: Ian Jackson <ian.jackson.citrix.com>