qemu-iotests/199: drop extra constraints

qemu-iotests/199: drop extra constraints

We don't need any specific format constraints here. Still keep qcow2
for two reasons:
1. No extra calls of format-unrelated test
2. Add some check around persistent bitmap in future (require qcow2)

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Andrey Shinkevich <andrey.shinkevich@virtuozzo.com>
Tested-by: Eric Blake <eblake@redhat.com>
Message-Id: <20200727194236.19551-3-vsementsov@virtuozzo.com>
Signed-off-by: Eric Blake <eblake@redhat.com>