e1000: flush packets when link comes up

e1000_can_receive() checks the link up status register bit.  If the bit
is clear, packets will be queued and the peer may disable receive to
avoid wasting CPU reading packets that cannot be delivered.  The queue
must be flushed once the link comes back up again.

This patch fixes broken e1000 receive with Mac OS X Snow Leopard guests
and tap networking.  Flushing the queue invokes the async send callback,
which re-enables tap fd read.

Reported-by: Jonathan Liu <net147@gmail.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Message-id: 1435223885-12745-1-git-send-email-stefanha@redhat.com
