This new listen type is currently supported only by spice graphics.
It's introduced to make it easier and clearer specify to not listen
anywhere in order to start a guest with OpenGL support.

The old way to do this was set spice graphics autoport='no' and don't
specify any ports.  The new way is to use <listen type='none'/>.  In
order to be able to migrate to old libvirt the migratable XML will be
generated without the listen element and with autoport='no'.  Also the
old configuration will be automatically converted to the this listen
type.
Signed-off-by: NPavel Hrdina <phrdina@redhat.com>