Comment 4 for bug 1820125

Reviewed: https://review.opendev.org/672746
Committed: https://git.openstack.org/cgit/openstack/nova/commit/?id=2b86a9caccb37c1800f3b5e6f4b29218a7539479
Submitter: Zuul
Branch: stable/rocky

commit 2b86a9caccb37c1800f3b5e6f4b29218a7539479
Author: Dan Smith <email address hidden>
Date: Thu Mar 14 14:07:31 2019 -0700

    Avoid crashing while getting libvirt capabilities with unknown arch names

    In _get_instance_capabilities() we get a list of host capabilities and then
    build a list of arches supported by the virt type of an instance to arrive
    at the list of possibilities for the instance. We check each of those
    against our enum, but fail to gracefully skip unsupported values should we
    encounter one.

    This patch makes that graceful, and also introduces an unsupported arch to
    the test stub to make sure we always skip it. Note that we do not warn
    because this happens once per instance in a periodic task, and since the
    situation is caused by a (somewhat permanent) mismatch of libvirt and
    nova version support, isn't something that needs to be remedied by an
    operator.

    Closes-Bug: #1820125
    Change-Id: I5d95bd50279a6bf903a5793ad5f3ae9d06f085f4
    (cherry picked from commit 71df650d0a390d2b6b19928db4379e723e9f5cde)