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
Lee Yarwood
0cfe9c81e3