migration: Create socket-address parameter

migration: Create socket-address parameter

It will be used to store the uri parameters. We want this only for
tcp, so we don't set it for other uris.  We need it to know what port
is migration running.

Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
  dgilbert: Removed DummyStruct as suggested by Eric & Markus

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