I had to reboot the host server but I did not want to stop one of my running VMs there. So what I did:
virsh save VM to save the vm state and restore it later with:
virsh restore VM. Every thing worked fine, so I could access my vm again. In the meanwhile I erased the file created when you run
virsh save since I thought (and I still think) that once the vm is restored is not needed anymore.
After one hour my VM froze and I did not manage to boot it again.