tests: link against libxml2

to avoid

CCLD     storagevolxml2argvtest
/usr/bin/ld: ../src/.libs/libvirt_driver_storage_impl.a(libvirt_driver_storage_impl_la-storage_backend.o): undefined reference to symbol 'xmlFreeDoc@@LIBXML2_2.4.30'
//usr/lib/x86_64-linux-gnu/libxml2.so.2: error adding symbols: DSO missing from command line

tests/Makefile.am

@@ -681,6 +681,7 @@ storagevolxml2argvtest_SOURCES = \
     storagevolxml2argvtest.c \
     testutils.c testutils.h
 storagevolxml2argvtest_LDADD = \
+	$(LIBXML_LIBS) \
 	../src/libvirt_driver_storage_impl.la $(LDADDS)
 
 else ! WITH_STORAGE