migration: Finer grained tracepoints for POSTCOPY_LISTEN

The enablement of postcopy listening has a few steps, add a few tracepoints to
be there ready for some basic measurements for them.

Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
Message-Id: <20220301083925.33483-3-peterx@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>