migration: Don't try and recover return path in non-postcopy

In normal precopy we can't do reconnection recovery - but we also
don't need to, since you can just rerun migration.
At the moment if the 'return-path' capability is on, we use
the return path in precopy to give a positive 'OK' to the end
of migration; however if migration fails then we fall into
the postcopy recovery path and hang.  This fixes it by only
running the return path in the postcopy case.

Reported-by: Greg Kurz <groug@kaod.org>
Tested-by: Greg Kurz <groug@kaod.org>
Reviewed-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>