We should never set permissions other than cumulative permissions of
parents. During bdrv_reopen_multiple() we _check_ for synthetic
permissions but when we do _set_ the graph is already updated.
Add an assertion to bdrv_reopen_multiple(), other cases are more
obvious.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20201106124241.16950-6-vsementsov@virtuozzo.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>