remote: Avoid coverity[leaked_storage] message.

Upon successful return of virNetClientStreamEventAddCallback() the
allocated cbdata field will be freed by virNetClientStreamEventRemoveCallback()
as cbOpaque using the free function remoteStreamCallbackFree().

src/remote/remote_driver.c

@@ -4816,6 +4816,7 @@ remoteStreamEventAddCallback(virStreamPtr st,
 
 cleanup:
     remoteDriverUnlock(priv);
+    /* coverity[leaked_storage] - cbdata is not leaked */
     return ret;
 }