vm-migration: Add start_migration() to Migratable trait

In order to clearly decouple when the migration is started compared to
when the dirty logging is started, we introduce a new method to the
Migratable trait. This clarifies the semantics as we don't end up using
start_dirty_log() for identifying when the migration has been started.
And similarly, we rely on the already existing complete_migration()
method to know when the migration has been ended.

A bug was reported when running a local migration with a vhost-user-net
device in server mode. The reason was because the migration_started
variable was never set to "true", since the start_dirty_log() function
was never invoked.

Signed-off-by: lizhaoxin1 <Lxiaoyouling@163.com>
Signed-off-by: Sebastien Boeuf <sebastien.boeuf@intel.com>

vm-migration/src/lib.rs

@@ -305,6 +305,10 @@ pub trait Migratable: Send + Pausable + Snapshottable + Transportable {
         Ok(MemoryRangeTable::default())
     }
 
+    fn start_migration(&mut self) -> std::result::Result<(), MigratableError> {
+        Ok(())
+    }
+
     fn complete_migration(&mut self) -> std::result::Result<(), MigratableError> {
         Ok(())
     }