VMs, and multiple disks priorities

I was missing virtual machines (XEN, VMware, ..) from the article, just
found above user comment mention it. Isn't the need for I/O controllers
very urgent when you start to use VMs? For a normal machine, if it's I/O
intensive you normally run only one application on it (database, webserver,
SAP) and don't really need control - just a faster disk subsystem.

But with multiple VMs running, the need to prioritize them grows, and if an
I/O controller gets into the kernel, it should help here. XENserver (which
is free since April) already allows priorities for VMs/disks, I wonder how
they implemented it?

Also, the other place where I see need of priorities for different mount
points. Each mount point should get a priority, so that you can say
/importantdb has higher priority than /standarddb. They could go to the
same disk subsystem, or another, but the thing is I want I/O on
/importantdb to not block because of /standarddb doing it's backup.