domain_conf: Free temporary variable

* src/conf/domain_conf.c: caller must free returned value of
  virXPathString in useserial

src/conf/domain_conf.c

@@ -5739,6 +5739,7 @@ virDomainDefParseBootXML(xmlXPathContextPtr ctxt,
         } else {
             def->os.bios.useserial = VIR_DOMAIN_BIOS_USESERIAL_NO;
         }
+        VIR_FREE(useserial);
     }
 
     *bootCount = deviceBoot;