Root-canal: Add missing payload size field to loopback evt

  std::unique_ptr<EventPacketBuilder> evt_ptr =

      std::unique_ptr<EventPacketBuilder>(new EventPacketBuilder(EventCode::LOOPBACK_COMMAND));

  CHECK(evt_ptr->AddPayloadOctets2(static_cast<uint16_t>(opcode)));

  CHECK(evt_ptr->AddPayloadOctets1(static_cast<uint8_t>(payload.size())));

  for (const auto& payload_byte : payload)  // Fill the packet.

    evt_ptr->AddPayloadOctets1(payload_byte);

  return evt_ptr;