vmm: Extract sparse file-copy helpers into a reusable module

Move next_data_extent and write_region_sparse out of memory_manager.rs
into a new vmm::sparse module so the snapshot writer, the restore
reader, and the offload daemon can share one implementation.

No functional change intended.

Signed-off-by: Sebastien Boeuf <sboeuf@meta.com>
Assisted-by: Claude:claude-opus-4-7

vmm/src/lib.rs

@@ -95,6 +95,7 @@ mod serial_manager;
 #[cfg(all(feature = "kvm", feature = "sev_snp", feature = "fw_cfg"))]
 pub(crate) mod sev;
 mod sigwinch_listener;
+pub mod sparse;
 mod sync_utils;
 mod uffd;
 mod userfaultfd;