target/riscv: Convert env->virt to a bool env->virt_enabled

target/riscv: Convert env->virt to a bool env->virt_enabled

Currently we only use the env->virt to encode the virtual mode enabled
status. Let's make it a bool type.

Signed-off-by: LIU Zhiwei <zhiwei_liu@linux.alibaba.com>
Reviewed-by: Weiwei Li <liweiwei@iscas.ac.cn>
Message-ID: <20230325145348.1208-1-zhiwei_liu@linux.alibaba.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20230327080858.39703-6-liweiwei@iscas.ac.cn>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>