slirp: Common lhost/fhost union

The socket structure has a pair of unions for lhost and fhost
addresses; the unions are identical so split them out into
a separate union declaration.

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org>