In the case of supporting V extension, add V extension description
to vmstate_riscv_cpu.

Signed-off-by: Yifei Jiang <jiangyifei@huawei.com>
Signed-off-by: Yipeng Yin <yinyipeng1@huawei.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-id: 20201026115530.304-6-jiangyifei@huawei.com
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>