disable xenlight for non-Xen platforms

when building libvirt in Fedora/s390x I've found that xenlight needs to
be explicitly disabled in the spec file. Configure properly sets the
library as non-existent, but the %files section still wants to package
the 3 /var/*/libvirt/libxl directories. See also
https://bugzilla.redhat.com/show_bug.cgi?id=745020

AUTHORS

@@ -196,6 +196,7 @@ Patches have also been contributed by:
   Lei Li               <lilei@linux.vnet.ibm.com>
   Matthias Witte       <witte@netzquadrat.de>
   Tang Chen            <tangchen@cn.fujitsu.com>
+  Dan Horák            <dan@danny.cz>
 
   [....send patches to get your name here....]
 

libvirt.spec.in

@@ -89,6 +89,7 @@
 # Xen is available only on i386 x86_64 ia64
 %ifnarch i386 i586 i686 x86_64 ia64
 %define with_xen 0
+%define with_libxl 0
 %endif
 
 # Numactl is not available on s390[x]