KVM: Enumerate support for PRIVATE memory iff kvm_arch_has_private_mem is defined

报告KVM_MEMORY_ATTRIBUTE_PRIVATE支持改为仅当kvm_arch_has_private_mem宏定义时，去除与CONFIG_KVM_VM_MEMORY_ATTRIBUTES的耦合，防止arm64在系统范围检查时误报PRIVATE支持。