I plan to build a server mainly for running virtual machines. I will use mainly Qemu(with KVM support) and Virtualbox as hypervisors. There will be dozens of virtual machines and they do not execute computationally expensive tasks. I am aware that I need to pick a CPU with VT-x/AMD-v support and I would benefit from Hyper-threading, but how do less high-frequency cores compare to more lower-frequency cores in hypervisor environment? Let's say for example quad-core Haswell microarchitecture Core i7-4790K where four cores run at 4GHz versus octa-core Xeon E5-1428L v3 where all eight cores run at 2GHz. If any additional information is required, then please ask.
This entirely depends on your needs, most would go for more cores as it means you may well be able to run more VMs, or VMs with more vCPUs, on a single host. But if you have a known number of VMs that would benefit from higher frequency CPUs then by all means go that way. That said I'd always prefer a Xeon over an iX CPU due to their additional reliability features.