migration: fix qemu_get_fd for BufferedFile

Not really used, but nice to have it correct. :)
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>
Signed-off-by: NJuan Quintela <quintela@redhat.com>

migration.c

@@ -625,7 +625,7 @@ static int buffered_get_fd(void *opaque)
 {
     QEMUFileBuffered *s = opaque;
 
-    return qemu_get_fd(s->file);
+    return s->migration_state->fd;
 }
 
 /*