migration: Fix possible race when setting rp_state.error

migration: Fix possible race when setting rp_state.error

We don't need to set the rp_state.error right after a shutdown because
qemu_file_shutdown() always sets the QEMUFile error, so the return
path thread would have seen it and set the rp error itself.

Setting the error outside of the thread is also racy because the
thread could clear it after we set it.

Reviewed-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Fabiano Rosas <farosas@suse.de>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-ID: <20230918172822.19052-3-farosas@suse.de>
(cherry picked from commit 28a8347281e24c2e7bba6d3301472eda41d4c096)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>