ocaml-xen-lowlevel-libs: don't even try to build xenguest

We have xenguest binaries for xen-4.2 and xen-4.4. Unfortunately
trunk seems to be neither of those (looking at the API)

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Merge pull request #75 from djs55/lowlevel.0.9.23

ocaml-xen-lowlevel-libs: don't even try to build xenguest