KVM: Enumerate support for PRIVATE memory iff kvm_arch_has_private_mem is defined

用 kvm_arch_has_private_mem 是否定义决定是否上报 KVM_MEMORY_ATTRIBUTE_PRIVATE，解除与 KVM_VM_MEMORY_ATTRIBUTES 耦合，避免 arm64 误报并准备 per-gmem 属性。