Enabling the 'allow_disk_format_probing = 1' in /etc/libvirt/qemu.conf and restarting libvirtd-bin re-instates the automated probing of backing_files in Maverick.
Enabling the 'allow_ disk_format_ probing = 1' in /etc/libvirt/ qemu.conf and restarting libvirtd-bin re-instates the automated probing of backing_files in Maverick.