Postcopy: Postcopy startup in migration thread

Rework the migration thread to setup and start postcopy.

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Amit Shah <amit.shah@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>
diff --git a/include/migration/migration.h b/include/migration/migration.h
index 2ad0d2b..ff13ff2 100644
--- a/include/migration/migration.h
+++ b/include/migration/migration.h
@@ -135,6 +135,9 @@
 
     /* Flag set once the migration has been asked to enter postcopy */
     bool start_postcopy;
+
+    /* Flag set once the migration thread is running (and needs joining) */
+    bool migration_thread_running;
 };
 
 void process_incoming_migration(QEMUFile *f);