Check for errors when parsing bridge interface XML

The return status of virInterfaceDefParseBridge() was not being
checked, potentially resulting in the creation of a broken
interface.
(cherry picked from commit cc004a66)

src/conf/interface_conf.c

@@ -769,7 +769,8 @@ virInterfaceDefParseXML(xmlXPathContextPtr ctxt, int parentIfType) {
             }
             def->data.bridge.delay = virXMLPropString(bridge, "delay");
             ctxt->node = bridge;
-            virInterfaceDefParseBridge(def, ctxt);
+            if (virInterfaceDefParseBridge(def, ctxt) < 0)
+                goto error;
             break;
         }
         case VIR_INTERFACE_TYPE_BOND: {