virtio: rng: s3/s4 support

Unregister from the hwrng interface and remove the vq before entering
the S3 or S4 states.  Add the vq and re-register with hwrng on restore.

Signed-off-by: Amit Shah <amit.shah@redhat.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>