RISC-V: KVM: selftests: Add steal_time test support

With the introduction of steal-time accounting support for
RISC-V KVM we can add RISC-V support to the steal_time test.

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>