net: add API to disable/enable polling

When vhost is bound to a backend device, we need to stop polling it when
vhost is started, and restart polling when vhost is stopped.
Add an API for that for use by vhost, and implement in tap backend.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>