migration/postcopy: Clear have_listen_thread

migration/postcopy: Clear have_listen_thread

Clear have_listen_thread when we exit the thread.
The fallout from this was that various things thought there was
an ongoing postcopy after the postcopy had finished.

The case that failed was postcopy->savevm->loadvm.

This corresponds to RH bug https://bugzilla.redhat.com/show_bug.cgi?id=1608765

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Message-Id: <20180914170430.54271-2-dgilbert@redhat.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>