What is a good way to monitor I/O individually for Xen Virtual Machines (not Dom0)?
You can use:
1) xentop (and drill down on I/O)
2) monitoring tools like xenmon (http://www.hpl.hp.com/techreports/2005/HPL-2005-187.html)
3) Standard I/O monitoring tools that you would normally use should give a good indication of the I/O usage of the guest as well.