ovirt: Don't try to use invalid port numbers
    
    If a remote oVirt VM don't specify a port/secure port number, we'd still
    try to pass it down to spice-gtk, which would then complain that 0 (the
    default value) isn't a valid port number.
    This commit make sure we filter out the default port/secure-port value
    and pass NULL to spice-gtk instead when we get these values.