When going into a Windows guest under Task Manager, Performance, the guest reports having a maximum of 4 cores and 4 vCPUs. This is inocorrect, as I had set 6 vcpus for the guest. (probably Windows 10 Enterprise has a max of 4 cores). Issue was seen on both UEFI and Legacy guests.
The issue was resolved in downstream by removing several patches, including cores-per-socket, hide-cores-, and other patches that mess with cpuid.
Dell XE3 and 7490
1. Install Windows guest with 6 vcpus.
2. Go to task manager, click on Performance. Notice that the Cores are 4.
Verified on both 1803 and 1809. Vcpus are listed as 6 in task manager after changing via command line. Closing.