partially_populate_device: align span positions to block size for better performance

[glehmann]

Add an optional align parameter (default: 4KiB) to align span positions
to a given block size. Only positions are aligned (rounded down); span sizes
are reduced if needed to avoid overlapping the next span's position. When
config.write_volume_cap is large enough, spans are contiguous and cover the
full device with no gaps. Without alignment, write throughput drops from
~700MiB/s to ~200MiB/s on large disks.

This PR is part of a tree containing 19 PRs:

1. master
2. storage: avoid removing the xva/vdi before going in the debugger #436 → master
3. storage: test large volumes #437 → storage: avoid removing the xva/vdi before going in the debugger #436
4. host: include mdadm RAID devices in disk detection #447 → storage: test large volumes #437
5. Update randstream to 0.5.0 #446 → host: include mdadm RAID devices in disk detection #447
6. storage: Avoid writing the whole device in coalesce tests #449 → Update randstream to 0.5.0 #446
7. storage: avoid writing the whole device in migration tests #450 → storage: Avoid writing the whole device in coalesce tests #449
8. xva/vdi: only write a small data amount in large volumes for faster tests #452 → storage: avoid writing the whole device in migration tests #450
9. storage: test full device write #453 → xva/vdi: only write a small data amount in large volumes for faster tests #452
10. storage: test that we can't create a vdi over its max allowed size #454 → storage: test full device write #453
11. storage: add jobs for large volume tests #461 → storage: test that we can't create a vdi over its max allowed size #454
12. skip large volume tests for zvol and nfsv4 #464 → storage: add jobs for large volume tests #461
13. storage: add comprehensive tests for lvmohba storage repositories #470 → skip large volume tests for zvol and nfsv4 #464
14. storage: free space for XVA import by destroying source VM first #471 → storage: add comprehensive tests for lvmohba storage repositories #470
15. storage: limit data written per VDI with --write-volume-cap #481 → storage: free space for XVA import by destroying source VM first #471
16. Add Packer configuration to build a minimal Alpine 3.23 UEFI VM for XCP-ng tests #523 → storage: limit data written per VDI with --write-volume-cap #481
17. migration: create a xfs sr on the second host for intra/cross-pool migration #497 → storage: limit data written per VDI with --write-volume-cap #481
18. Enhance block device management #498 → migration: create a xfs sr on the second host for intra/cross-pool migration #497
19. Add VHD_MAX and QCOW2_MAX symbolic size constants #500 → Enhance block device management #498
20. "partially_populate_device: align spans to block size for better performance" (this PR) → Add VHD_MAX and QCOW2_MAX symbolic size constants #500

[Millefeuille42]

I'd rather have the argument called block_size. Align makes it ambiguous whether it's a flag or a value.

[glehmann]

done!