feat(@xen-orchestra/backups): use parallel reading whith NBD + blocks to only one remote

[fbeauchamp]

Description

PR #6716 removed the parallel reading in favor a widespread use of NBD and unique reading

this PR make it possible to use parallel reading given theses prequisites :

• delta backup job
• only one remote AND it is store vhd in block
• no replication

Checklist

• Commit
  • Title follows commit conventions
  • Reference the relevant issue (Fixes #007, See xoa-support#42, See https://...)
  • If bug fix, add Introduced by
• Changelog
  • If visible by XOA users, add changelog entry
  • Update "Packages to release" in CHANGELOG.unreleased.md
• PR
  • If UI changes, add screenshots
  • If not finished or not tested, open as Draft

[fbeauchamp]

no real performance gain after tests

[olivierlambert]

Darn, so we don't use NBD read on multiple blocks at the same time anymore? I'm surprised there's no diff vs our initial tests, but I might be confused about the purpose of this PR.