Add tests for three security flaws wrt handling of qcow2 backing
files. This ensures that libvirtd does not automatically probe
disk formats ever, and correctly creates qcow2 files with backing
formats.

* scripts/qemu/200-qcow2-single-backing-file.t: Check that
  libvirt honours the format in the guest <disk> driver type.
* scripts/qemu/205-qcow2-double-backing-file.t: Check that
  libvirt honours the format embedded in the qcow2 backing
  store format extension
* scripts/storage/250-vol-qcow2-backing-store.t: Check that
  the backing store format extension is set
* scripts/qemu/210-qcow2-auto-probing.t,
  scripts/storage/255-vol-qcow2-backing-store-auto.t: Check
  that any unspecified disk formats default to raw.