virtio-serial driver bugfixes

[Martinits]

Fix 2 migtd virtio-serial driver bugs:

• Missing control msgs since qemu may put multiple control msg but the driver pops one each time.
• A late PORT_OPEN event may cause a PortNotAvailable when port.open(), try to drain control queue when PortNotAvailable and retry open.

[jyao1]

could you please fix CI failure?

[Martinits]

@jyao1 CI failures fixed.

[sgrams]

Two small things to address before we can merge this:

1. recv_control_nonblock is missing the dma_allocation.get -> safe_len = min(vq_buf.len, record.dma_size) clamp that recv_control performs. Both paths consume a host-controlled vq_buf.len; the clamp is a deliberate defense against an overlong len. Worth mirroring so both helpers are equally hardened.
2. Retry-exhaustion path silently returns Ok(port) - if all 100 retries fail with PortNotAvailable, the for loop falls through and setup_transport returns an unopened port. The failure then surfaces at the first send/recv with no breadcrumb. Track success with a bool and return Err(VirtioSerialError::PortNotAvailable(VIRTIO_SERIAL_PORT_ID)) + a log::error! on exhaustion.

@Martinits can you please help to address it? It should be a 3-line change, see below:

For 1)

            for vq_buf in &h2g {
                if let Some(record) = self.dma_allocation.get(&vq_buf.addr) {
                    let safe_len = core::cmp::min(vq_buf.len as usize, record.dma_size);
                    let control_msg = unsafe {
                        core::slice::from_raw_parts(vq_buf.addr as *const u8, safe_len)
                    };
                    self.handle_control_msg(control_msg)?;
                    self.free_dma_memory(vq_buf.addr)
                        .ok_or(VirtioSerialError::OutOfResource)?;
                }
            }

For 2)

        let port = VirtioSerialPort::new(VIRTIO_SERIAL_PORT_ID);
        let mut opened = false;
        for _ in 0..VIRTIO_SERIAL_OPEN_RETRY_TIMES {
            match port.open() {
                Ok(()) => {
                    opened = true;
                    break;
                }
                Err(VirtioSerialError::PortNotAvailable(_)) => {
                    // `PORT_OPEN` can arrive after init_control; pump control queue during retry.
                    // If we drained pending control msgs, try open again immediately to avoid
                    // waiting for the next retry tick.
                    Timer::after(VIRTIO_SERIAL_OPEN_RETRY_DELAY).await;
                    VirtioSerial::try_poll_control()?;
                }
                Err(e) => return Err(e.into()),
            }
        }
        if !opened {
            log::error!(
                "virtio-serial port {} not available after {} retries ({}ms total)\n",
                VIRTIO_SERIAL_PORT_ID,
                VIRTIO_SERIAL_OPEN_RETRY_TIMES,
                VIRTIO_SERIAL_OPEN_RETRY_TIMES * VIRTIO_SERIAL_OPEN_RETRY_DELAY.as_millis() as usize,
            );
            return Err(VirtioSerialError::PortNotAvailable(VIRTIO_SERIAL_PORT_ID).into());
        }
        return Ok(port);

[sgrams]

See my previous comment