vhost: send SET_VRING_ENABLE at start/stop

Send SET_VRING_ENABLE at start/stop, to give the backend
an explicit sign of our state.

Signed-off-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>