virtio-rng: ask for more data if queue is not fully drained

This commit effectively reverts:

  commit 4621c1768ef5d12171cca2aa1473595ecb9f1c9e
  Author: Amit Shah <amit.shah@redhat.com>
  Date:   Wed Nov 21 11:21:19 2012 +0530

  virtio-rng: remove extra request for entropy

but instead of calling virtio_rng_process unconditionally, it
first checks to see if the queue is empty as a little bit of
optimization.

Signed-off-by: Ladi Prosek <lprosek@redhat.com>
Reviewed-by: Amit Shah <amit.shah@redhat.com>
Message-Id: <1456998514-19271-1-git-send-email-lprosek@redhat.com>
Signed-off-by: Amit Shah <amit.shah@redhat.com>
