KVM: arm64: selftests: Rename vcpu_config and add to kvm_util.h

Rename vcpu_config to vcpu_reg_list to be more specific and add
it to kvm_util.h. While it may not get used outside get-reg-list
tests, exporting it doesn't hurt, as long as it has a unique enough
name. This is a step in the direction of sharing most of the get-
reg-list test code between architectures.

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