test: only attempt to read and write sectors which exist
test: only attempt to read and write sectors which exist

If a disk is 0 length tnen no sectors exist.

We still need some negative tests to check that such attempts
fail properly.

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

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