Detect qcow2 images by reading the image file header. It contains the
"QFI" string and the qcow version number. The libvirt template is
configured to use the qcow2 driver when a qcow2 image is detected.
The root partition is altered by attaching it to a host device using
qemu-nbd. Since qemu-nbd can be named differently depending on the
Linux distribution, it is configurable by mount.conf.