fix(backups): handle slow enable cbt

[fbeauchamp]

when VDI.enable_cbt is too slow to answer we may take the snapshot before cbt is enabled, it fails silently , then take the snapshot , then on the next backup cbt is enabled , but it fails because cbt started after the last snapshot, but since cbt was enabled, the data are purged, leading to a new full backup

from 29965

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