conf: Run devicePostParse() again for the first serial device
    
    The devicePostParse() callback is invoked for all devices so that
    drivers have a chance to set their own specific values; however,
    virDomainDefAddImplicitDevices() runs *after* the devicePostParse()
    callbacks have been invoked and can add new devices, in which case
    the driver wouldn't have a chance to customize them.
    
    Work around the issue by invoking the devicePostParse() callback
    after virDomainDefAddImplicitDevices(), only for the first serial
    devices, which might have been added by it. The same was already
    happening for the first video device for the very same reason.
    
    This will become important later on, when we will change
    virDomainDefAddConsoleCompat() not to set a targetType for
    automatically added serial devices.
    
    Signed-off-by: Andrea Bolognani <abologna@redhat.com>
    Reviewed-by: Pavel Hrdina <phrdina@redhat.com>