iotests: don't test qcow2.py inside 291

iotests: don't test qcow2.py inside 291

820c6bee534ec3b added testing of qcow2.py into 291, and it breaks 291
with external data file. Actually, 291 is bad place for qcow2.py
testing, better add a separate test.

For now, drop qcow2.py testing from 291 to fix the regression.

Fixes: 820c6bee534ec3b
Reported-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20200618154052.8629-1-vsementsov@virtuozzo.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>