random(4) masks unregistered entropy sources. Prior to this revision,
virtio_random(4) did not correctly register a random_source and did not
function as a source of entropy.
Random source registration for loadable pure sources requires registering a
poll callback, which is invoked periodically by random(4)'s harvestq
kthread. The periodic poll makes virtio_random(4)'s periodic entropy
collection redundant, so this revision removes the callout.
Add locking around concurrent access to the device; the virtqueue use
depends on the assumption that only a single reader is accessing the device
at a time.