libqos/virtio.c: Correct 'flags' reading in qvirtqueue_kick

libqos/virtio.c: Correct 'flags' reading in qvirtqueue_kick

In qvirtqueue_kick(), the 'flags' were previously being incorrectly read from
vq->avail instead of the correct vq->used location. This update ensures 'flags'
are read from the correct location as per the virtio standard.

Signed-off-by: Zheyu Ma <zheyuma97@gmail.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-ID: <20240320090442.267525-1-zheyuma97@gmail.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>