feat(nbd-client/multi): harden NBD backups

[fbeauchamp]

review by commit

Description

• NBD client tried to connect to all NBD enabled network, even the network non accessible by xoa or proxy

this PR implement :

• a parallel retry that should speed up connection when a lot of unreachable NBD network exists and improve reliability, since failing to connect to NBD was leading to a fallback in vdi.exportContent, fallback that was not working with purgeData
• filtering network passed to nbd-client to ensure "default backup network" from the pool setting is respected
• if it's not possible to produce a delta ( no changed blocks but can't compute changed blocks) : make error clearer

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