Xspice isn't very useful without the TCP connection possibility. When the port is busy:
$ Xspice --disable-ticketing --xsession /usr/bin/gnome-terminal :0 --port 5900
(/usr/bin/Xspice:6677): Spice-Warning **: reds.c:3037:reds_init_socket: reds_init_socket: binding socket to :5900 failed
If the port was explicitly requested, failure to bind should be prominently displayed, and preferably result in a termination of Xspice.
qemu bug about the same: https://bugzilla.redhat.com/show_bug.cgi?id=682227
Author: Marc-André Lureau <email@example.com>
Date: Mon Nov 3 01:14:48 2014 +0100
Xspice: fail if server can't be initialized