vmm: migration: emit event for each memory iteration

Emit a "vm.migration-memory-iteration" event after every precopy memory
iteration to allow management software to observe forward progress
during migration.

This event is primarily intended for integration with management
software such as libvirt, where it maps to
VIR_DOMAIN_EVENT_ID_MIGRATION_ITERATION.

The event is intentionally independent of any upcoming migration
metrics endpoint. Detailed migration statistics will be exposed via
that endpoint, while this event provides a lightweight progress signal
expected by external management layers.

With this event, management software can detect forward progress during
migration without being blocked on any upcoming migration metrics
endpoint.

Signed-off-by: Philipp Schuster <philipp.schuster@cyberus-technology.de>
On-behalf-of: SAP philipp.schuster@sap.com

vmm/src/lib.rs

@@ -1283,6 +1283,14 @@ impl Vmm {
             // Log progress of the current iteration
             debug!("Precopy: {ctx}");
 
+            // Enables management software (e.g., libvirt) to easily track forward progress.
+            event!(
+                "vm",
+                "migration-memory-iteration",
+                "id",
+                ctx.iteration.to_string()
+            );
+
             // Increment iteration last: This way we ensure that the logging
             // above matches the actual iteration.
             ctx.iteration += 1;