iotests: dump QCOW2 header in JSON in #303

Extend the test case #303 by dumping QCOW2 image metadata in JSON
format.

Signed-off-by: Andrey Shinkevich <andrey.shinkevich@virtuozzo.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <1596742557-320265-12-git-send-email-andrey.shinkevich@virtuozzo.com>
Signed-off-by: Eric Blake <eblake@redhat.com>
2 files changed