Free the macvtap mode string

* src/conf/domain_conf.c: forgot to free the attribute value in
  virDomainNetDefParseXML()

src/conf/domain_conf.c

@@ -1974,6 +1974,7 @@ cleanup:
     VIR_FREE(type);
     VIR_FREE(internal);
     VIR_FREE(devaddr);
+    VIR_FREE(mode);
 
     return def;