This post was very helpful to me. I was able to get mine going with a slight modification of your code (as my host path was a serial path not a tty:
<serial type='dev'>
<source path='/dev/serial/by-id/<yourusbid>'/> <----note this is the host path
<target type='usb-serial' port='1'>
<model name='usb-serial'/>
</target>
<alias name='serial1'/>
<address type='usb' bus='0' port='4'/>
</serial>
I deploy through a VM on unRAID.