Can not resize to same host for libvirt driver

[Eric Xie]

Description
===========
As before, the instance can be resized or cold-migrated to same host for libvirt driver
if CONF.allow_resize_to_same_host was set to true.
When use latest source, got error "UnableToMigrateToSelf: Unable to migrate instance".

Steps to reproduce
==================
* Configure CONF.allow_resize_to_same_host to true
* Create one instance
* Cold-migrate the instance

Expected result
===============
The instance can be cold-migrated on same host

Actual result
=============
Got error "UnableToMigrateToSelf: Unable to migrate instance"

Environment
===========
$ git log
commit f571151e79dbd87a76ae3222a9f5b507d85648b1
Merge: 3233392 236f1b2
Author: Zuul <email address hidden>
Date: Sat May 30 06:55:18 2020 +0000

    Merge "zuul: Make devstack-plugin-ceph-tempest-py3 a voting check job again"

libvirt + KVM

Logs & Configs
==============
[DEFAULT]
allow_resize_to_same_host = true

nova-compute
2020-06-01 06:53:24.367 28545 ERROR nova.compute.manager [instance: 982f9273-eb50-443a-8bbc-fa728ceac8e4] UnableToMigrateToSelf: Unable to migrate instance (982f9273-eb50-443a-8bbc-fa728ceac8e4) to current host (compute04).

[Balazs Gibizer (balazs-gibizer)]

Which nova version you used where cold migrate worked for the same host? By looking at the code I think since Liberty the libvirt driver explicitly state it does not support cold migration to the same host [1]. Also I'm not sure what would be the reason to migrate an instance to the same host with libvirt. (In VMware where a single compute host manages multiple hypervisors it could make sense).

In the other hand resize to same host supported with libvirt driver and works for me in a fresh devstack.

I'm marking this bug Invalid cold migrate to the same host is not supported by libvirt driver. Please set it back to New if you disagree.

[1] https://review.opendev.org/#/c/118604/20/nova/virt/libvirt/driver.py