conf: Introduce VIR_DOMAIN_CHR_SERIAL_TARGET_TYPE_NONE
    
    This is the first step in getting rid of the assumption that
    isa-serial is the default target type for serial devices.
    
    Signed-off-by: Andrea Bolognani <abologna@redhat.com>
    Reviewed-by: Pavel Hrdina <phrdina@redhat.com>