Update to Cohttp.0.12.0 API
Update to Cohttp.0.12.0 API

This is the API in [mirage/ocaml-cohttp#196]

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

euanh/vhd-tool,thomassa/vhd-tool,djs55/vhd-tool,djs55/vhd-tool,euanh/vhd-tool