virtio: rng: don't wait on host when module is going away

No use waiting for input from host when the module is being removed.
We're going to remove the vq in the next step anyway, so just perform
any other steps for cleanup (currently none).

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