net: fix qemu_flush_queued_packets() in presence of a hub

net: fix qemu_flush_queued_packets() in presence of a hub

When frontend and backend are connected through a hub as below
(showing only one direction), and the frontend (or in general, all
output ports of the hub) cannot accept more traffic, the backend
queues packets in queue-A.

When the frontend (or in general, one output port) becomes ready again,
quemu tries to flush packets from queue-B, which is unfortunately empty.

  e1000.0 <--[queue B]-- hub0port0(hub)hub0port1 <--[queue A]-- tap.0

To fix this i propose to introduce a new function net_hub_flush()
which is called when trying to flush a queue connected to a hub.

Signed-off-by: Luigi Rizzo <rizzo@iet.unipi.it>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>