virtio_net: flush uncompleted TX on reset
    
    If the backend could not transmit a packet right away for some reason,
    the packet is queued for asynchronous sending. The corresponding vq
    element is tracked in the async_tx.elem field of the VirtIONetQueue,
    for later freeing when the transmission is complete.
    
    If a reset happens before completion, virtio_net_tx_complete() will push
    async_tx.elem back to the guest anyway, and we end up with the inuse flag
    of the vq being equal to -1. The next call to virtqueue_pop() is then
    likely to fail with "Virtqueue size exceeded".
    
    This can be reproduced easily by starting a guest with an hubport backend
    that is not connected to a functional network, eg,
    
     -device virtio-net-pci,netdev=hub0 -netdev hubport,id=hub0,hubid=0
    
    and no other -netdev hubport,hubid=0 on the command line.
    
    The appropriate fix is to ensure that such an asynchronous transmission
    cannot survive a device reset. So for all queues, we first try to send
    the packet again, and eventually we purge it if the backend still could
    not deliver it.
    
    CC: qemu-stable@nongnu.org
    Reported-by: R. Nageswara Sastry <nasastry@in.ibm.com>
    Buglink: https://github.com/open-power-host-os/qemu/issues/37
    Signed-off-by: Greg Kurz <groug@kaod.org>
    Tested-by: R. Nageswara Sastry <nasastry@in.ibm.com>
    Signed-off-by: Jason Wang <jasowang@redhat.com>
    (cherry picked from commit 94b52958b77a2a040564cf7ed716d3a9545d94e5)
    Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>