Update to the new ocaml-vhd API 'Vhd_IO.openchain'

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
vhd-tool info: only open the given filename, rather than the whole chain

The whole chain is not needed if all we are trying to do
is read (or write) metadata.

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Merge pull request #10 from djs55/openchain

Adapt to new ocaml-vhd API