The geometry calculation needs to use int64s
The geometry calculation needs to use int64s

A ~2 TiB disk will contain 41-bit sector offsets

This fixes the ~2 TiB unit tests and exposes a problem with the
0-sized tests.

Signed-off-by: David Scott <63c9eb0ea83039690fefa11afe17873ba8278a56@eu.citrix.com>

djs55/ocaml-vhd,xapi-project/ocaml-vhd