bhyve: support random number generator device

bhyve: support random number generator device

Bhyve supports the Virtio RNG interface. It's always using the
/dev/random device and doesn't have any configuration options.

Thus, in XML it's represented as:

  <rng model='virtio'>
    <backend model='random'/>
  </rng>

So extend the bhyve driver to support that and add a set of tests for
this feature.

Signed-off-by: Roman Bogorodskiy <bogorodskiy@gmail.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>