[CentOS] CentOS 5.6 and KVM failure

After updating to 5.6 on a server this morning, I can no longer boot two
virtual machines. One is trixbox which I believe is a 32bit centos based
distro, and the other vm is a 64bit Windows 2008 installation.

The error I get in the virt-manager console is FATAL: No bootable device.

Both VMs are qcow2 format, and I've checked file permissions. I'm going to
boot a live cd in the trixbox vm to see if I can access anything on it's
qcow2 disk.

Any hints appreciated...