@sapcode Hi there --
I know this was quite a while ago but I thought I should post here since I ran into the same exact issue when trying to add >64 vCPUs. The issue ended up being an ACPI issue on the VM itself. Things apparently get wonky past 64 CPUs with ACPI, since VMs well don't really need ACPI you can disable and voila, more than 64 vCPUs :]
Just modify your VM's platform param, no need to pass it as a kernel option inside the VM.
xe vm-param-set platform:acpi=0
Hope this helps anyone else trying to run more than 64 vCPUs.
cc @jeff