You must pin them to specific cpu cores. But then you lose a big part of the convenience offered by VMs. It is like using gpu passthrough vs virtual 3d card.
Not that big part at all, you can change these settings at will. Sharing the CPUs (overcommiting) is probably out of the question though.
Just most VM management UIs lack the functionality.
GPU passthrough is special because they have weird initialization and shutdown handling. Plus are chunky (all our nothing) until SR-IOV variants become commonplace.
CPU cores don't have these problems.