tools/virtio: Add --batch=random option

So we can test with non-deterministic batches in flight.

Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
Link: https://lore.kernel.org/r/20200418102217.32327-4-eperezma@redhat.com


Signed-off-by: Michael S. Tsirkin <mst@redhat.com>