vhd-format-lwt: fix build with OCaml 4.03
vhd-format-lwt: fix build with OCaml 4.03

Local opening of modules in a pattern has been introduced in OCaml 4.04.

Signed-off-by: Gabor Igloi <7535f5501fd71876c93d896691e178fd3a11cfbb@citrix.com>

djs55/ocaml-vhd