ocaml: eventchn: add a 'type t' to represent an event channel

ocaml: eventchn: add a 'type t' to represent an event channel

It's a common OCaml convention to add a 'type t' in a module to
represent the main "thing" that the module is about. We add an
opaque type t and to_int/of_int functions for those who really
need it, in particular:

  1. to_int is needed for debug logging; and
  2. both to_int and of_int are needed for anyone who communicates
     a port number through xenstore.

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