virtio-net: stop/start bh when appropriate

Avoid sending out packets, and modifying
memory, when VM is stopped.
Add assert statements to verify this does not happen.

Avoid scheduling bh when vhost-net is started.

Stop bh when driver disabled bus mastering
(we must not access memory after this).

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Tested-by: Jason Wang <jasowang@redhat.com>
