migration: make qemu_fopen_ops_buffered() return void

We want the file assignment to happen before the thread is created to
avoid locking, so we just do it before creating the thread.

Signed-off-by: Juan Quintela <quintela@redhat.com>

Reviewed-by: Orit Wasserman <owasserm@redhat.com>

include/migration/migration.h

@@ -45,6 +45,7 @@ struct MigrationState
     int64_t dirty_pages_rate;
     bool enabled_capabilities[MIGRATION_CAPABILITY_MAX];
     int64_t xbzrle_cache_size;
+    bool complete;
 };
 
 void process_incoming_migration(QEMUFile *f);