qcow2: Version 3 images

This adds the basic infrastructure to qcow2 to handle version 3 images.
It includes code to create v3 images, allow header updates for v3 images
and checks feature bits.

It still misses support for zero clusters, so this is not a fully
compliant implementation of v3 yet.

The default for creating new images stays at v2 for now.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
