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.

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
ocaml: eventchn: in the interface, we don't have to give implementation details

Remove the mention of the C function names from the .mli -- this is only
needed in the implementation .ml

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
ocaml: eventchn: add ocamldoc strings to the interface

Also add a reference to tools/libxc/xenctrl.h, which is where
the underlying C functions are defined.

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
ocaml: eventchn: expose bindings for 'init' and 'fd' in the interface

These are needed to actually open the event channel interface and
to get a file descriptor suitable for Unix.select.

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
ocaml: eventchn: remove the unused exception 'Eventchn.Error'

Signed-off-by: David Scott <dave.scott@eu.citrix.com>