This patch adds support for the qcow2 image format to blktap. It
consists mostly of qemu code, adapted to the blktap interfaces.
Snapshots and compressed images are supported.

The qcow2 driver may be used by either specifying tap:qcow2 or by
using tap:qcow which will detect that you have a version 2 image and
will call the qcow2 driver.

Signed-off-by: Kevin Wolf <kwolf@suse.de>