Fix some build errors on xen-4.1
Fix some build errors on xen-4.1

euanh/ocaml-xen-lowlevel-libs,djs55/ocaml-xen-lowlevel-libs,jonludlam/ocaml-xen-lowlevel-libs,djs55/ocaml-xen-lowlevel-libs,djs55/ocaml-xen-lowlevel-libs,euanh/ocaml-xen-lowlevel-libs,djs55/ocaml-xen-lowlevel-libs,euanh/ocaml-xen-lowlevel-libs