Fix determination of mirage-xen location.

This fixes an issue related to #100 in mirage-xen.  As you can see, I'm delaying the ocamlfind query, so that we don't need to guess which of potentially multiple library directories will contain mirage-xen once it gets installed.   If you don't like this, I think it would be okay to just substitute `ocamlfind printconf path` with `ocamlfind printconf destdir`.