fix(backups): really delete vhd/xva when deleting backups

[fbeauchamp]

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
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[fbeauchamp]

Why did I let from afterBackup of MixinBackup and did I not create a scheduledCleanVm method in RemoteAdapter

• the declaration of cleanVm in MIxinBackup will be duplicated, since it's still needed by beforeBackup and will be needed by scheduledCleanVm
• afterBackup needs lockFile, which is not needed for deleteVm
• the config for disableMergeWorker does not seems easily accessible from RemoteAdapter
• deporting this to a MergeWorker means the backup will not really deleted when the UI mark it as done
• without Merge, the main cost of the cleanVm script will be the deleting, we should be quite fast for vhdFile ( but slow for vhdDirectory).

[julien-f]

IMHO, this PR contains 3 different fixes (or two fixes and a feature/chore), I would expect 3 separate commits.