There is no disk.local for VMs having ephemeral in flavor 

Description
===========

This is not really a functionality issue because the VM did get created, the ephemeral disk is visible in the VM and there is a disk.eph0.

But there is a code in driver checking for disk.local if ephemeral disk is added for disk_mapping,
https://github.com/openstack/nova/blob/a9b1e024eb1e8ddd286174bd576a64ff224a9221/nova/virt/libvirt/driver.py#L4794

there is no local.disk in disk_mapping or real disk inside data/instances/<instance-id>/ directory. It do not get created during resize process either.

Reason:

build_and_run_instance: compute.manager
    # block_device_mapping received here is bdm object.
    build an object having image, device/disk and network info called resources
    with resources ASK driver to spawn VM
                 # resources = block_device_info: {disk, ephemeral, image }, network_info = {}

                 get_disk_mapping from libvirt/blockinfo
#https://github.com/openstack/nova/blob/17b46bae84bef38bc27976219a244010f1e510e3/nova/virt/libvirt/blockinfo.py#L714

                driver.spawn runs _create_image
                # pass disk_maping to _create_image
                creates instance directory
                creates 2: disk.info and disk
                look for disk.local in disk_mapping
                # but its not present so disk.local hence did not get created in compute under compute node.

Steps to reproduce
==================

- openstack flavor create --ram 512 --disk 20 --ephemeral 15 --vcpus 1 102
- openstack server create --flavor 102 --network private --image cirros-0.6.2-x86_64-disk vm_3

Expected result
===============
- there should be a disk.local in directory
  data/nova/instances/<instance-id>/

or during resize

Actual result
=============
- disk.local is NOT present in instance/<instance-id> directory
- disk.eph0 is present instance/<instance-id> directory
- there is a data file ephemeral_15_40d1d2c inside instance/_base

Environment
===========
tested in branch master and stable/train

So basically for which scenarios disk.local should/will get created?