add back trace

examples/x86_64/src/tcp.rs

@@ -92,6 +92,7 @@ impl<T: Transport> TxToken for VirtioTxToken<T> {
         let mut dev = self.0.borrow_mut();
         let mut tx_buf = dev.new_tx_buffer(len);
         let result = f(tx_buf.packet_mut());
+        trace!("SEND {} bytes: {:02X?}", len, tx_buf.packet());
         dev.send(tx_buf).unwrap();
         result
     }