configure: add a --disable-xenguest argument
configure: add a --disable-xenguest argument

For distributions for which we cannot build any sensible xenguest, disable
it entirely and use libxl.

Signed-off-by: David Scott <63c9eb0ea83039690fefa11afe17873ba8278a56@citrix.com>

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