CP-10188: Package xentoollog as a top-level library

This pull request needs to be done in tandem with similar branches in ocaml-xen-lowlevel-libs and xen-api-libs-specs

https://github.com/xapi-project/ocaml-xen-lowlevel-libs/pull/73
https://github.com/xenserver/xen-api-libs-specs/pull/93
Hmm - I failed when shuffling branches