target/riscv: Handle KVM_EXIT_RISCV_SBI exit

target/riscv: Handle KVM_EXIT_RISCV_SBI exit

Use char-fe to handle console sbi call, which implement early
console io while apply 'earlycon=sbi' into kernel parameters.

Signed-off-by: Yifei Jiang <jiangyifei@huawei.com>
Signed-off-by: Mingwang Li <limingwang@huawei.com>
Reviewed-by: Anup Patel <anup.patel@wdc.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-id: 20220112081329.1835-9-jiangyifei@huawei.com
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>