See Get the CPU Pinning Settings for a Guest
Pinning is a way to limit the effect a certain guest has on others, or to give a guest a dedicated core etc.
To set the cores that a guest can use.
virsh vcpupin VM_ID 0 2
NOTE: This will set the first vCPU (the one with ID 0) to only run on core ID 2.
Thus the output of **virsh vcpupin VM_ID would report:
VCPU: CPU Affinity
0: 2 1: 0-3