virtio-rng: Remove blocking /dev/urandom

It's a bad idea to claim to support /dev/urandom but rely on HW RNG
because starting up Cassandra, for example, takes ages.

Drop it until we have a cryptographically secure PRNG in OSv that can be
used to implement /dev/urandom properly.

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
Signed-off-by: Avi Kivity <avi@cloudius-systems.com>