[CentOS] KVM problem after update to 5.6

Hello, after updating to Cents 5.6 and so to kvm-83-224
my KVM virtual machines qemu qcow2 based images do not start anymore.
Looking at VM console the error message is that VM media is not bootable.
Going back to previous KVM version kvm-83-164 from Centos 5.5 they works 
again.

What's wrong with qemu images ?
anyone has an idea on how to fix the problem ?

thanks

Rick