Comment 5 for bug 1864020

Reviewed: https://review.opendev.org/710788
Committed: https://git.openstack.org/cgit/openstack/nova/commit/?id=7fd41e9b8c0f0f9cbccd63fbdbcc863de7db50f5
Submitter: Zuul
Branch: stable/train

commit 7fd41e9b8c0f0f9cbccd63fbdbcc863de7db50f5
Author: Lee Yarwood <email address hidden>
Date: Wed Feb 19 20:39:54 2020 +0000

    libvirt: Provide the backing file format when creating qcow2 disks

    Libvirt v6.0.0 [1] will now fail to launch a domain when using qcow2
    disks where the backing file format is not recorded in the qcow2
    metadata.

    There are some discussions upstream around relaxing this slightly [2]
    but for now any attempt to launch an instance using qcow2 disks will
    fail as Nova does not populate this value when creating the disk.

    Nova needs to at a minimum start populating this field and depending on
    the outcome of the thread upstream in Libvirt also potentially handle
    the upgrade case where we may need to rebase existing disks in order to
    update the metadata.

    For now this change simply adds the backing_fmt option to the qemu-img
    command line used to create these disks.

    [1] https://github.com/libvirt/libvirt/commit/3615e8b39badf2a526996a69dc91a92b04cf262e
    [2] https://www.redhat.com/archives/libvir-list/2020-February/msg00616.html

    Partial-Bug: #1864020
    Change-Id: I77ebada015f6522a300be4fa043fb8676458402b
    (cherry picked from commit 0cfe9c81e3fe4d268c9949c6b8e873abab94a407)