virtio-rng: add a control virtqueue

[vivier]

If for some reasons the host cannot provide enough entropy to the device, the driver is stuck waiting for the enqueued request buffers. Because of that, we cannot switch to another entropy device or to stop and unload the driver.

We need a way to retrieve these buffers and release all the driver resources.

Adding a control virtqueue with a command to flush the request virtqueue seems to be the way to do it.

[vivier]

Proposed changes to the specifications:
0001-virtio-rng-add-a-control-queue.txt

[vivier]

Proposed implementation:
QEMU: https://github.com/vivier/qemu/commits/virtio-rng-ctrl
linux: https://github.com/vivier/linux/commits/virtio-rng-ctrl