[incomplete] xen: use common Event, Grant code from ocaml-xen-lowlevel-libs

The OCaml code is shared in ocaml-xen-lowlevel-libs, all the differences exist in the C stubs: mirage-xen calls hypercalls, userspace uses libxc. This makes it possible to parallel install mirage-xen and ocaml-xen-lowlevel-libs, previously they conflicted over the ocamlfind package 'xenctrl'

Pull together with
* [mirage/mirage-console#4]
* [mirage/mirage-net-xen#5]
* [mirage/mirage-block-xen#17]
* [xapi-project/ocaml-xen-lowlevel-libs#43]