The block job holds a reference to the backup-top node (because it is
passed as the main job BDS to block_job_create()).  Therefore,
bdrv_backup_top_drop() cannot delete the backup-top node (replacing it
by its child does not affect the job parent, because that has
.stay_at_node set).  That is a problem, because all of its I/O functions
assume the BlockCopyState (s->bcs) to be valid and that it has a
filtered child; but after bdrv_backup_top_drop(), neither of those
things are true.

It does not make sense to add new parents to backup-top after
backup_clean(), so we should detach it from the job before
bdrv_backup_top_drop().  Because there is no function to do that for a
single node, just detach all of the job's nodes -- the job does not do
anything past backup_clean() anyway.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-Id: <20210219153348.41861-2-mreitz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>