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

hw/virtio-pci.h

@@ -17,6 +17,7 @@
 
 #include "virtio-blk.h"
 #include "virtio-net.h"
+#include "virtio-rng.h"
 #include "virtio-serial.h"
 #include "virtio-scsi.h"
 
@@ -46,6 +47,7 @@ typedef struct {
     virtio_serial_conf serial;
     virtio_net_conf net;
     VirtIOSCSIConf scsi;
+    VirtIORNGConf rng;
     bool ioeventfd_disabled;
     bool ioeventfd_started;
     VirtIOIRQFD *vector_irqfd;