vm-migration: Snapshot should have a unique SnapshotDataSection

There's no reason to carry a HashMap of SnapshotDataSection per
Snapshot. And given we now provide at most one SnapshotDataSection per
Snapshot, there's no need to keep the id part of the SnapshotDataSection
structure.

Signed-off-by: Sebastien Boeuf <sebastien.boeuf@intel.com>

vmm/src/vm.rs

@@ -2522,7 +2522,6 @@ impl Snapshottable for Vm {
 
         vm_snapshot.add_snapshot(self.device_manager.lock().unwrap().snapshot()?);
         vm_snapshot.add_data_section(SnapshotDataSection {
-            id: format!("{}-section", VM_SNAPSHOT_ID),
             snapshot: vm_snapshot_data,
         });