virtio-rng: hardware random number generator device

virtio-rng: hardware random number generator device

The Linux kernel already has a virtio-rng driver, this is the device
implementation.

When the guest asks for entropy from the virtio hwrng, it puts a buffer
in the vq.  We then put entropy into that buffer, and push it back to
the guest.

Signed-off-by: Amit Shah <amit.shah@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
---
aliguori: converted to new RngBackend interface
aliguori: remove entropy needed event
aliguori: fix migration