virtio-rng: disable timer on device removal

Disable the rate-limit timer on device remove (e.g. hot-unplug).

Signed-off-by: Amit Shah <amit.shah@redhat.com>
Reviewed-by: Anthony Liguori <aliguori@us.ibm.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
