qcow2.py: Introduce '-j' key to dump in JSON format

qcow2.py: Introduce '-j' key to dump in JSON format

Add the command key to the qcow2.py arguments list to dump QCOW2
metadata in JSON format. Here is the suggested way to do that. The
implementation of the dump in JSON format is in the patch that follows.

Signed-off-by: Andrey Shinkevich <andrey.shinkevich@virtuozzo.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <1596742557-320265-9-git-send-email-andrey.shinkevich@virtuozzo.com>
Signed-off-by: Eric Blake <eblake@redhat.com>