Moved mainloop to OCaml

This makes the code simpler and easier to explain.
Fix querying of location of mirage-xen.

The issue arise when multiple paths have been specified in findlib.conf.
- For installation use destdir instead of path.
- In scripts/mir-xen-build, use ocamlfind query mirage-xen.
Merge branch 'ocaml-mainloop' of git://github.com/talex5/mirage-platform into talex5-ocaml-mainloop
Sync CHANGES
Merge branch 'fix-mirage-xen-destdir' of git://github.com/paurkedal/mirage-platform
sync CHANGES