Add fw_cfg table for loongarch virt machine, including memmap table.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Xiaojuan Yang <yangxiaojuan@loongson.cn>
Message-Id: <20220712083206.4187715-2-yangxiaojuan@loongson.cn>
[rth: Replace fprintf with assert; drop unused return value;
      initialize reserved slot to zero.]
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>