hwrng: virtio - add an internal buffer

hwrng: virtio - add an internal buffer

hwrng core uses two buffers that can be mixed in the
virtio-rng queue.

If the buffer is provided with wait=0 it is enqueued in the
virtio-rng queue but unused by the caller.
On the next call, core provides another buffer but the
first one is filled instead and the new one queued.
And the caller reads the data from the new one that is not
updated, and the data in the first one are lost.

To avoid this mix, virtio-rng needs to use its own unique
internal buffer at a cost of a data copy to the caller buffer.

Signed-off-by: Laurent Vivier <lvivier@redhat.com>
Link: https://lore.kernel.org/r/20211028101111.128049-2-lvivier@redhat.com
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>