Update ocaml-vhd to 0.6.1

Signed-off-by: Si Beaumont <simon.beaumont@citrix.com>
CA-120439: Update to vhd-tool 0.6.1

Signed-off-by: Si Beaumont <simon.beaumont@citrix.com>
CA-120439: Update to vhd-tool 0.6.2

Signed-off-by: Si Beaumont <simon.beaumont@citrix.com>