I got the serial problem fixed by doing the following:

   

    ./ linux ubd0=root_fs_cow ssl5=tty:/dev/ttyS0

    UML#  echo "Hello" > /dev/tts/5 (earlier was using /dev/ttys/5)

 

This works assuming the baud-rate and terminal attributes for the HOST ttySx are in sync.

 

Cheers!