Ignore listen attribute of <graphics> for type network listens

Commit 69929941 started filling the listen attribute
of the parent <graphics> elements from type='network' listens.

When this XML is passed to UpdateDevice, parsing fails:
XML error: graphics listen attribute 10.20.30.40 must match
address attribute of first listen element (found none)

Ignore the address in the parent <graphics> attribute
when no type='address' listens are found,
the same we ignore the address for the <listen> subelements
when parsing inactive XML.

src/conf/domain_conf.c

@@ -9614,12 +9614,16 @@ virDomainGraphicsDefParseXML(xmlNodePtr node,
                         break;
                     }
                 }
-                if (!matched) {
+                if (found && !matched) {
                     virReportError(VIR_ERR_XML_ERROR,
                                    _("graphics listen attribute %s must match address "
                                      "attribute of first listen element (found %s)"),
-                                   listenAddr, found ? found : "none");
+                                   listenAddr, found);
                     goto error;
+                } else if (!found) {
+                    /* quietly ignore listen address if none of the listens
+                     * are of type address */
+                    VIR_FREE(listenAddr);
                 }
             }
         }