KVM: Enumerate support for PRIVATE memory iff kvm_arch_has_private_mem is defined

仅当架构定义 kvm_arch_has_private_mem() 时才上报 KVM_MEMORY_ATTRIBUTE_PRIVATE，避免 guest_memfd 属性通用化后 arm64 等系统级误报；与 CONFIG_KVM_VM_MEMORY_ATTRIBUTES 解耦。