CA-120525: Update to vhd-tool/ocaml-vhd 0.6.3

Signed-off-by: Si Beaumont <simon.beaumont@citrix.com>
Update to vhd-tool 0.6.4

Signed-off-by: Si Beaumont <simon.beaumont@citrix.com>
CA-120549: Update to ocaml-vhd 0.6.4

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