KVM Host with 88 physical cores uses only 14 on 64 core Guest
There was a vCPU pinning in the XML which doesn't appear in virt-manager -.- I removed it via
for i in {0..63}; do sudo virsh vcpupin $kvmdomain $i r --config; done
and removed the line in the XML-file via
sudo virsh edit <domain>
After that I restarted the guest.