qcow2_format.py: make printable data an extension class member

qcow2_format.py: make printable data an extension class member

Let us differ binary data type from string one for the extension data
variable and keep the string as the QcowHeaderExtension class member.

Signed-off-by: Andrey Shinkevich <andrey.shinkevich@virtuozzo.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <1596742557-320265-3-git-send-email-andrey.shinkevich@virtuozzo.com>
Signed-off-by: Eric Blake <eblake@redhat.com>