KVM: arm64: selftests: Move reject_set check logic to a function

No functional changes. Just move the reject_set check logic to a
function so we can check for a specific errno. This is a preparation
for support reject_set in riscv.

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>