Link libvirt_xenconfig instead of libvirt against libxl

to avoid the test failure

 7) Test driver "xen"  ... 2016-03-31 12:53:26.950+0000: 22430: debug : virDriverLoadModule:54 : Module load xen
 2016-03-31 12:53:26.950+0000: 22430: error : virDriverLoadModule:73 : failed to load module /build/libvirt-1.3.3~rc1/debian/build/src/.libs/libvirt_driver_xen.so /build/libvirt-1.3.3~rc1/debian/build/src/.libs/libvirt_driver_xen.so: undefined symbol: xlu_cfg_destroy
FAILED

(cherry picked from commit 12b239a9)

src/Makefile.am

@@ -1134,7 +1134,7 @@ endif WITH_VMX
 if WITH_XENCONFIG
 noinst_LTLIBRARIES += libvirt_xenconfig.la
 libvirt_la_BUILT_LIBADD += libvirt_xenconfig.la
-libvirt_la_LIBADD += $(LIBXL_LIBS)
+libvirt_xenconfig_la_LIBADD = $(LIBXL_LIBS)
 libvirt_xenconfig_la_CFLAGS = \
 		-I$(srcdir)/conf $(AM_CFLAGS)
 libvirt_xenconfig_la_SOURCES = $(XENCONFIG_SOURCES)