virtio-devices: console: Use 0 as the 'len' for the transmit queue

For a transmit queue (guest to host), the host only reads from the
guest-provided buffers and does not write to them. According to the
virtio specification (e.g., Section 2.6.8 in Virtio 1.1), the 'len'
field in the used ring has to be set to the number of bytes written to
the buffers. Therefore, it should be 0 for the console transmit queue.

Signed-off-by: Andrei Vagin <avagin@google.com>

virtio-devices/src/console.rs

@@ -265,7 +265,7 @@ impl ConsoleEpollHandler {
                 out.flush().map_err(Error::OutputFlush)?;
             }
             trans_queue
-                .add_used(desc_chain.memory(), desc_chain.head_index(), desc.len())
+                .add_used(desc_chain.memory(), desc_chain.head_index(), 0)
                 .map_err(Error::QueueAddUsed)?;
             used_descs = true;
         }