net: invoke callback when purging queue

devices rely on packet callbacks eventually running,
but we violate this rule whenever we purge the queue.
To fix, invoke callbacks on all packets on purge.
Set length to 0, this way callers can detect that
this happened and re-queue if necessary.

Cc: qemu-stable@nongnu.org
Cc: Jason Wang <jasowang@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
