virtio-net: fix TX timer with tx_burst

virtio-net: fix TX timer with tx_burst

When virtio_net_flush_tx() reaches the tx_burst value all
the queue is not flushed and nothing restart the timer.

Fix that by doing for TX timer as we do for bottom half TX:
rearming the timer if we find any packet to send during the
virtio_net_flush_tx() call.

Fixes: e3f30488e5f8 ("virtio-net: Limit number of packets sent per TX flush")
Cc: alex.williamson@redhat.com
Signed-off-by: Laurent Vivier <lvivier@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>