Create sparse files on backup import and migration receive

[milaiwi]

What Changed

1. incusd/storage/drivers: Introduce SparseFileWrapper

• Added SparseFileWrapper struct and Write function to internal/server/storage/drivers/utils.go

2. incusd/storage/drivers/vfs: Use SpraseFileWrapper on backup import

• Changed the io.Copy call in genericVFSBackupUnpack to use our wrapper struct.

Examples

• On Creation

ubuntu@ubuntu:~/incus$ sudo du -sch /var/lib/incus/storage-pools/dir/virtual-machines/a1/root.img
254M	/var/lib/incus/storage-pools/dir/virtual-machines/a1/root.img
254M	total
ubuntu@ubuntu:~/incus$ sudo  sha256sum /var/lib/incus/storage-pools/dir/virtual-machines/a1/root.img
3621514d973e5e7f80f7c9302d1348bd2a9789454421ec3f458bac7c0feb9f0c  /var/lib/incus/storage-pools/dir/virtual-machines/a1/root.img

• On Import

ubuntu@ubuntu:~/incus$ sudo du -sch /var/lib/incus/storage-pools/dir/virtual-machines/a1/root.img
254M	/var/lib/incus/storage-pools/dir/virtual-machines/a1/root.img
254M	total
ubuntu@ubuntu:~/incus$ sudo  sha256sum /var/lib/incus/storage-pools/dir/virtual-machines/a1/root.img
3621514d973e5e7f80f7c9302d1348bd2a9789454421ec3f458bac7c0feb9f0c  /var/lib/incus/storage-pools/dir/virtual-machines/a1/root.img

Closes #662

[stgraber]

Thanks, I've just done a few minor edits:

• Move the struct and function together so it's easier to find
• Tweaked the comments on the struct and function to comply with godoc format
• Made the W field private (renamed to w)
• Added a NewSparseFileWrapper function that can be used externally (as w is now private)
• Simplified the wrapping in genericVFSBackupUnpack
• Added the same wrapping to genericVFSCreateVolumeFromMigration