conf: Drop virDomainChrDeviceType.targetTypeAttr
    
    This attribute was used to decide whether to format the type
    attribute of the <target> element, but the logic didn't take into
    account all possible cases and as such could lead to unexpected
    results. Moreover, it's one more thing to keep track of, and can
    easily fall out of sync with other attributes.
    
    Now that we have VIR_DOMAIN_CHR_SERIAL_TARGET_TYPE_NONE, we can
    use that value to signal that no specific target type has been
    configured for the serial device and as such the attribute should
    not be formatted at all. All other values are now formatted.
    
    Signed-off-by: Andrea Bolognani <abologna@redhat.com>
    Reviewed-by: Pavel Hrdina <phrdina@redhat.com>