hw/riscv: virt: Exit if the user provided -bios in combination with KVM

hw/riscv: virt: Exit if the user provided -bios in combination with KVM

The -bios option is silently ignored if used in combination with -enable-kvm.
The reason is that the machine starts in S-Mode, and the bios typically runs in
M-Mode.

Better exit in that case to not confuse the user.

Signed-off-by: Ralf Ramsauer <ralf.ramsauer@oth-regensburg.de>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Anup Patel <anup@brainfault.org>
Message-Id: <20220401121842.2791796-1-ralf.ramsauer@oth-regensburg.de>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>