Comment 10 for bug 656173

Updated suggested release note text (my last one had a typo and I've tried to address Jamin's concerns):

Previous libvirt versions would probe a qemu disk to determine its format and did not require that the format be declared in the XML. This is considered a security problem in most deployments and newer versions of libvirt will default to the 'raw' format when the format is not specified in the XML. As a result, non-raw disks without a specified disk format will no longer be available in
existing virtual machines.

The libvirt-migrate-qemu-disks tool is provided to aid in transitioning virtual machine definitions to the new required format. In essence, it will check all domains for affected virtual machines, probe the affected disks and update the domain definition accordingly. This command will be run automatically on upgrade. For new virtual machines using non-raw images, the disk format must be specified in the domain XML provided to libvirt, otherwise the disk will not be available to the virtual machine. See 'man 1 libvirt-migrate-qemu-disks' for details.

Users who require the old behavior can adjust the 'allow_disk_format_probing' option in /etc/libvirt/qemu.conf.