network: bridge: Fix regression when defining persistent networks

Commit 0211fd6e04cdc402da20818df54299c6ded3d3cb introduced regression
where newly defined networks were not made persistent.

This patch makes the network persistent on each successful definition.

src/network/bridge_driver.c

@@ -3124,6 +3124,9 @@ static virNetworkPtr networkDefine(virConnectPtr conn, const char *xml) {
             goto cleanup;
     }
 
+    /* define makes the network persistent - always */
+    network->persistent = 1;
+
     /* def was asigned */
     freeDef = false;