[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-console
* mirage-net-xen
* mirage-block-xen
* ocaml-xen-lowlevel-libs