[CentOS] KVM problem after update to 5.6

Mon Apr 11 09:30:38 UTC 2011
Riccardo Veraldi <Riccardo.Veraldi at cnaf.infn.it>

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 

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