KVM: riscv: selftests: Add riscv vm satp modes

KVM: riscv: selftests: Add riscv vm satp modes

Current vm modes cannot represent riscv guest modes precisely, here add
all 9 combinations of P(56,40,41) x V(57,48,39). Also the default vm
mode is detected on runtime instead of hardcoded one, which might not be
supported on specific machine.

Signed-off-by: Wu Fei <wu.fei9@sanechips.com.cn>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Reviewed-by: Nutty Liu <nutty.liu@hotmail.com>
Reviewed-by: Anup Patel <anup@brainfault.org>
Link: https://lore.kernel.org/r/20251105151442.28767-1-wu.fei9@sanechips.com.cn
Signed-off-by: Anup Patel <anup@brainfault.org>