Fix xlconfigtest with older libxl

Commit cd5dc303 added this test, but it fails if
LIBXL_HAVE_BUILDINFO_USBDEVICE_LIST is not defined:

6) Xen XM-2-XML Format fullvirt-multiusb
... libvirt:  error : unsupported configuration: multiple USB
devices not supported
FAILED

tests/xlconfigtest.c

@@ -215,7 +215,10 @@ mymain(void)
 
     DO_TEST("new-disk", 3);
     DO_TEST("spice", 3);
+
+#ifdef LIBXL_HAVE_BUILDINFO_USBDEVICE_LIST
     DO_TEST("fullvirt-multiusb", 3);
+#endif
 
     virObjectUnref(caps);
     virObjectUnref(xmlopt);