Update to use new Tar, Tar_unix interface from ocaml-tar 0.2.1

This must be merged in combination with: [xapi-project/stdext#3] and [xapi-project/xen-api-libs-specs#20]

The new ocaml-tar library separates the modules into:

   Tar: for pure OCaml definitions
   Tar_unix: for impure Unix I/O
   Tar_lwt: for impure Lwt I/O

So mostly we have to replace Tar. with Tar_unix. and adjust the
OCAMLPACKS.

Most of this code concerns VM import/export. There is an alternative
implementation which is much more efficient inside recent releases
of vhd-tool, so we should switch to that soon.