Always search for parents relative to the directory containing the leaf
Always search for parents relative to the directory containing the leaf

It is nolonger necessary for clients (such as sparse_dd) to pass in the
dirname of a vhd leaf: we will always interpret ".." "./foo/bar" in a
parent locator as relative to the current directory of the file containing
the locator.

This fixes the broken unit test.

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

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