i have exactly the same issue and would love to hear how to do it without setting up VM from scratch :)
I tried to restart VM turn it off and change nothing helps still CPU is set to custom. I tried Host Model and Host passthru. Maybe there is no information (XML) regarding my CPU Intel Xeon E5-2640v2 and that is why it reverts to Custom. What's more i even cannot change assigned CPU/Cores/Threads to the VM, maybe there is something broken in latest Cobian release??