conf: fix migratable XML for graphics if socket is generated based on config
    
    The graphics code is complex and there are a lot of exceptions and
    backward compatible combinations.  One of them is the possibility
    to configure "spice_auto_unix_socket" in qemu.conf which will convert
    all spice graphics with listen type "address" without any address
    specified to listen type "socket" when the guest is started.
    
    We don't format this generated socket into migratable XML to make
    migration work with older libvirt.  However, spice has another
    exception that if autoport='no' and there is no port configured
    it is converted to listen type "none".  Because of this we need
    to format autoport='yes' to make sure that the listen type will
    be the same as the offline XML.
    
    Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1511407
    
    Reviewed-by: Jiri Denemark <jdenemar@redhat.com>
    Signed-off-by: Pavel Hrdina <phrdina@redhat.com>