virtio-rng: add rate limiting support

This adds parameters to virtio-rng-pci to allow rate limiting the entropy a
guest receives.  An example command line:

$ qemu -device virtio-rng-pci,max-bytes=1024,period=1000

Would limit entropy collection to 1Kb/s.

Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
