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).

Wouldn't something like 'legacy' be more descriptive than 'oss' (here and in a few other places)?