Allow max concurrency per VDI/disk in backup jobs

[ghost]

It would be very valuable to be able to set concurrency for backing up the individual VDIs/disks within one backup job. This is especially important for remotes and SR's that use spinning disks drives.

Example:
I have set concurrency for backup jobs to 1 but disks within each job are still transferred in parallel.

The VM has 4 disks assigned and all disks are transferred in parallel, which is eating up unnecessary I/O. The remote storage for the backups is using spinning disks, so concurrent I/O is not optimal.

[fbeauchamp]

there is a diskPerVmConcurrencyimplemented by #6787 for this ( not exposed in the UI for now)

[olivierlambert]

Good enough to me 👍 Closing!