Delta generation fails for file based SRs

Context:

• file based SR (local file or NFS)
• delta asked between 2 snapshots of a disk

Issue:

• create a VDI snapshot
• grow the VDI
• create a new snapshot of this same VDI
• ask a delta between those 2 snapshots

Result: VDI I/O error, 100% guaranteed.

LVM based SRs are not affected.

Let me know if you need more log details, but it's very easy to reproduce.