This patch introduces self_announce_delay() to calculate the delay for
the next announce round. This could be used by other device e.g
virtio-net who wants to do announcing by itself.

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