virtio: document virtio_config_ops restrictions

Some transports (e.g. virtio-ccw) implement virtio operations that
seem to be a simple read/write as something more involved that
cannot be done from an atomic context.

Give at least a hint about that.

Signed-off-by: Cornelia Huck <cohuck@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>