virtio-rng: add check of period

If period is assigned to 0, limit timer will expire immediately.
It causes a qemu warning:

"main-loop: WARNING: I/O thread spun for 1000 iterations"

This limit is meaningless. This patch forbids to assign 0 to period.

Reviewed-by: Amit Shah <amit.shah@redhat.com>
Signed-off-by: Amos Kong <akong@redhat.com>
Message-id: 1385031203-23790-1-git-send-email-akong@redhat.com
Signed-off-by: Anthony Liguori <aliguori@amazon.com>
