spice: do not require TCP ports

It is possible to use Spice server without TCP port.  On local VM,
qemu (and libvirt) can add new clients thanks to QMP add_client command.

Signed-off-by: Marc-André Lureau <marcandre.lureau@gmail.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>