Replace virStreamFree with virObjectUnref

Since virStreamFree will call virObjectUnref anyway, let's just use that
directly so as to avoid the possibility that we inadvertently clear out
a pending error message when using the public API.

daemon/remote.c

@@ -5414,7 +5414,7 @@ remoteDispatchDomainMigratePrepareTunnel3Params(virNetServerPtr server ATTRIBUTE
             virStreamAbort(st);
             daemonFreeClientStream(client, stream);
         } else {
-            virStreamFree(st);
+            virObjectUnref(st);
         }
     }
     return rv;