RISC-V: KVM: selftests: Add guest_sbi_probe_extension

Add guest_sbi_probe_extension(), allowing guest code to probe for
SBI extensions. As guest_sbi_probe_extension() needs
SBI_ERR_NOT_SUPPORTED, take the opportunity to bring in all SBI
error codes. We don't bring in all current extension IDs or base
extension function IDs though, even though we need one of each,
because we'd prefer to bring those in as necessary.

Reviewed-by: Anup Patel <anup@brainfault.org>
Reviewed-by: Atish Patra <atishp@rivosinc.com>
Signed-off-by: Andrew Jones <ajones@ventanamicro.com>
Signed-off-by: Anup Patel <anup@brainfault.org>