CP-8988: Support environments without suspend image conversion tools

This has been tested on a https://github.com/xenserver/buildroot environment running with a version of xenguest supporting the `-supports` option (https://github.com/xapi-project/ocaml-xen-lowlevel-libs/commit/6495760c0a711e10fe8428d3a9057999910ae057).

Yes, well maybe something more like `legacy_libxc_detected` would be more descriptive. It's more symmetric with the types in `Suspend_image` even if it is a bit more to type :)