tools/virtio: Reset index in virtio_test --reset.

This way behavior for vhost is more like a VM.

Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
Link: https://lore.kernel.org/r/20200418102217.32327-8-eperezma@redhat.com


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