There are still few uses of direct xenstore access, most of them are
xen-specific (so doesn't need to be portable). For now simply don't
connect to xenstore when no 'xen.lowlevel.xs' module present. It will
break such xen-specific accesses - it must be somehow reworked - either
by adding appropriate conditionals, or moving such code somewhere else
(custom methods of libvirt driver?).