PCI devices cannot be detached if all the PCI devices are claimed when resize

[Claudiu Belu]

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

PCI devices are requested through flavor extra_specs, and they can de attached / detached by resizing to a flavor having more / fewer PCI devices in the extra_specs.

But, if all the PCI devices have been claimed, they cannot be detached anymore as cold resize will fail.

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

* Let's say we have 2 PCI devices whitelisted on a compute node.
* Create a flavor which requires both PCI devices.
* Create an instance with said flavor.
* Resize the instance to a flavor without any PCI devices.

Expected result
===============

The instance should be resized, and have no PCI devices attached.

Actual result
=============

The instance is in ERROR state, the cold resize failed.

Logs & Configs
==============

[1] resize attempt, n-api.log, n-sch.log: http://paste.openstack.org/show/598884/
[2] #openstack-nova discussion: http://eavesdrop.openstack.org/irclogs/%23openstack-nova/%23openstack-nova.2017-02-14.log.html#t2017-02-14T19:45:02