My experience for using the virtio drivers on a heavy loaded production system is that it is not stable enough:
Testing it a week I am getting several times remarkable slow down of the virtual server (Windows SBS 2003 Standard,Windows 2003 Standard with Blackberry Enterprise Server). Only shutdown and restart could solve this. One time a complete shutdown of two server. Maybe due to heavy load through the backup processes. At the moment I won't use these drivers.