feat(backups): replication failing with snapshot attached to dom0

[fbeauchamp]

disks could still be atached to dom0 after the transfers given the caches and the asynchronous nature of the operation.

This PR ensure the cache are fully up to date before running the destroy code and also will let the XAPI handle the disk attached to dom0, as it was before the refactoring

Description

Short explanation of this PR (feel free to re-use commit message)

Checklist

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