replication: Properly attach children

replication: Properly attach children

The replication driver needs access to the children block-nodes of
it's child so it can issue bdrv_make_empty() and bdrv_co_pwritev()
to manage the replication. However, it does this by directly copying
the BdrvChilds, which is wrong.

Fix this by properly attaching the block-nodes with
bdrv_attach_child() and requesting the required permissions.

This ultimatively fixes a potential crash in replication_co_writev(),
because it may write to s->secondary_disk if it is in state
BLOCK_REPLICATION_FAILOVER_FAILED, without requesting write
permissions first. And now the workaround in
secondary_do_checkpoint() can be removed.

Signed-off-by: Lukas Straub <lukasstraub2@web.de>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <5d0539d729afb8072d0d7cde977c5066285591b4.1626619393.git.lukasstraub2@web.de>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>