add revert to work around performance regression when backing up large RBD disk

add revert to work around performance regression when backing up large RBD disk

resulting in QMP timeouts and very slow backups. The plan is to figure
out (ideally together with upstream) a way to make the implementation
of bdrv_co_block_status for RBD more efficient. But for now, revert
the problematic change as a stop-gap measure.

Upstream bug report:
https://gitlab.com/qemu-project/qemu/-/issues/1026

Forum threads:
https://forum.proxmox.com/threads/109272/
https://forum.proxmox.com/threads/109448/
https://forum.proxmox.com/threads/101334/ (partially)

Signed-off-by: Fabian Ebner <f.ebner@proxmox.com>