vm-migration: add migration-level context for downtime tracking

Add migration-level context types that extend the existing memory-only
metrics with overall migration duration and downtime breakdown.

OngoingMigrationContext models the sender-side migration progress until
all inputs needed for final downtime accounting are available.
CompletedMigrationContext then stores the finalized migration metrics,
including the final memory iteration, snapshotting, snapshot transfer,
and completion phase.

This provides the data needed to log effective downtime in the VMM and
lays the groundwork for future migration statistics reporting.

# Terminology

At first glance, the use of "state" and "[VM] snapshot" may seem
confusing. As discussed in [0], we use "state" consistently in the
migration code. On the VM side, "snapshotting" is merely the mechanism
used to obtain the VM state.

[0] https://github.com/cloud-hypervisor/cloud-hypervisor/pull/7979#discussion_r3061359899

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

vm-migration/src/protocol.rs

@@ -115,6 +115,7 @@ pub enum Command {
     Config,
     State,
     Memory,
+    /// Finalizes the migration and resumes the VM on the guest.
     Complete,
     Abandon,
     MemoryFd,