rpc: fix a typo in debugging log in virNetServerProgramSendStreamData

The bufferOffset has been initialized to zero in virNetMessageEncodePayloadRaw(),
so, we use bufferLength to represent the length of message which is going to be
sent to client side.

src/rpc/virnetserverprogram.c

@@ -468,7 +468,7 @@ int virNetServerProgramSendStreamData(virNetServerProgramPtr prog,
         if (virNetMessageEncodePayloadEmpty(msg) < 0)
             return -1;
     }
-    VIR_DEBUG("Total %zu", msg->bufferOffset);
+    VIR_DEBUG("Total %zu", msg->bufferLength);
 
     return virNetServerClientSendMessage(client, msg);
 }