This adds the virtio-rng driver to OSv.  The implementation is simple:

  - Start a thread that keeps 64 byte of entropy cached in internal
    buffer.  Entropy is gathered from the host with virtio-rng.

  - Create device nodes for "/dev/random" and "/dev/urandom" that both
    use the same virtio_rng_read() hook.

  - Use the entropy buffer for virtio_rng_read().  If we exhaust the
    buffer, wake up the thread and wait for more entropy to appear.

We eventually should move device node creation to separate
drivers/random.c that multiplexes between different hardware RNG
implementations.  However, as we only support virtio-rng, I'm leaving
that to whomever implements support for the next RNG.

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