qemu-iotests: qcow2.py

qemu-iotests: qcow2.py

This adds a tool that is meant to inspect and edit qcow2 files in a
low-level way, that wouldn't be possible with qemu-img/io, for example
by adding yet unknown extensions or flags. This way we can test whether
qemu deals properly with future backwards compatible extensions.

For now, let's start with the image header and header extensions.

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