fix(Restore): ignore cbt_medata disk when doing differential restores

[fbeauchamp]

differential restore works by reusing local data of a snapshot linked to a delta chain on the remote cbt_metadata don't have the disks data , thus are not eligible to be used for differential restores

from https://xcp-ng.org/forum/topic/9728/problem-with-differential-restore/8

Description

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Checklist

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• develop skills of junior reviewers
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2. Review process:
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