iscsi: don't leak portal string when starting a pool

(cherry picked from commit 413274f6)

src/storage/storage_backend_iscsi.c

@@ -735,6 +735,7 @@ virStorageBackendISCSIStartPool(virConnectPtr conn ATTRIBUTE_UNUSED,
     ret = 0;
 
 cleanup:
+    VIR_FREE(portal);
     VIR_FREE(session);
     return ret;
 }