KVM: riscv: selftests: Use register subtypes

Always use register subtypes in the get-reg-list test when registers
have them. The only registers neglecting to do so were ISA extension
registers. While we don't really need to use KVM_REG_RISCV_ISA_SINGLE
(since it's zero), the main purpose is to avoid confusion and to
self-document the tests. Also add print support for the multi
registers like SBI extensions have, even though they're only used for
debugging.

Signed-off-by: Andrew Jones <ajones@ventanamicro.com>
Reviewed-by: Haibo Xu <haibo1.xu@intel.com>
Reviewed-by: Anup Patel <anup@brainfault.org>
Signed-off-by: Anup Patel <anup@brainfault.org>