migration: fix incorrect enable return path

0425dc9 is actually v1 of that patch, but it was accidentally
merged (while there was a v2). That will cause problem when we try to
migrate to some old QEMUs when return path is not really there. Let's
fix it, then squashing this patch with 0425dc9 will be exactly patch
content of v2.

Fixes: 0425dc9 ("migration: isolate return path on src")
Signed-off-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>

migration/migration.c

@@ -1814,7 +1814,11 @@ static void *migration_thread(void *opaque)
 
     qemu_savevm_state_header(s->to_dst_file);
 
-    if (s->to_dst_file) {
+    /*
+     * If we opened the return path, we need to make sure dst has it
+     * opened as well.
+     */
+    if (s->rp_state.from_dst_file) {
         /* Now tell the dest that it should open its end so it can reply */
         qemu_savevm_send_open_return_path(s->to_dst_file);