Libvirt 6.0.0 requires the backing file format for qcow2

[Dmitriy Rabotyagov]

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

Upgraded deployments of Ubuntu Bionic to Ussuri have issue with rebooting instances, that are using QCOW2 images. While new instances don't have this issue thanks to https://review.opendev.org/c/openstack/nova/+/708745 but the ones, that were created before upgrade, are not able to reboot

Solution
========

The issue has been fixed with libvirt 6.1.0, with patch https://github.com/libvirt/libvirt/commit/ae9e6c2a2b

However, max libvirt package provided by UCA for Bionic is 6.0.0 Updating lib.
EL has overcome the issue for their distros by using libvirt 6.1.0