Don't build libxenlight driver for Xen 4.0

The libxenlight driver does not build against the tech preview
version of libxenlight in Xen 4.0.  Only enable building the
driver against more complete libxenlight found in Xen 4.1.

configure.ac

@@ -512,7 +512,7 @@ if test "$with_libxl" != "no" ; then
     fi
     CFLAGS="$CFLAGS $LIBXL_CFLAGS"
     LIBS="$LIBS $LIBXL_LIBS"
-    AC_CHECK_LIB([xenlight], [libxl_ctx_init], [
+    AC_CHECK_LIB([xenlight], [libxl_domain_create_new], [
         with_libxl=yes
         LIBXL_LIBS="$LIBXL_LIBS -lxenlight -lxenstore -lxenctrl -lxenguest -luuid -lutil -lblktapctl"
     ],[