ocaml: Add alternate object-oriented programming style.

ocaml: Add alternate object-oriented programming style.

In original style:

let () =
  let filename = Sys.argv.(1) in
  let g = Guestfs.create () in
  Guestfs.add_drive_ro g filename;
  Guestfs.launch g;
  let roots = Guestfs.inspect_os g in
  print_endline (Guestfs.inspect_get_product_name g roots.(0))

The same code in the new OO style:

let () =
  let filename = Sys.argv.(1) in
  let g = new Guestfs.guestfs in
  g#add_drive_ro filename;
  g#launch ();
  let roots = g#inspect_os () in
  print_endline (g#inspect_get_product_name roots.(0))