net: virtio-net discards TX data after link down

https://bugzilla.redhat.com/show_bug.cgi?id=1295637
Upon set_link monitor command or upon netdev deletion
virtio-net sends link down indication to the guest
and stops vhost if one is used.
Guest driver can still submit data for TX until it
recognizes link loss. If these packets not returned by
the host, the Windows guest will never be able to finish
disable/removal/shutdown.
Now each packet sent by guest after NIC indicated link
down will be completed immediately.

Signed-off-by: Yuri Benditovich <yuri.benditovich@daynix.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
