KVM: selftests: Only do get/set tests on present blessed list

Only do the get/set tests on present and blessed registers
since we don't know the capabilities of any new ones.

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