fix(backups,vhd-lib): merge with VhdSynthetic

[fbeauchamp]

introduced by #6184

test recipe I followed :

• make backups to have a chain of at least 4 vhds
• make a 5th backup, it should merge , no warning during cleanup
• reduce backup retention by 2
• merge should occurs, only 2 backup should be restorable
• make a new backup

ensure there is changed data between backup, for exemaple with a reboot

use two remote : one with vhdfile and another with vhd directory

Check list

Check if done, if not relevant leave unchecked.

• PR reference the relevant issue (e.g. Fixes #007 or See xoa-support#42)
• if UI changes, a screenshot has been added to the PR
• documentation updated
• CHANGELOG.unreleased.md:
  • enhancement/bug fix entry added
  • list of packages to release updated (${name} v${new version})
• I have tested added/updated features (and impacted code)
  • unit tests (e.g. cron/parse.spec.js)
  • if xo-server API changes, the corresponding test has been added to/updated on xo-server-test
  • at least manual testing

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