libvirt: add support for per-vCPU pinning in guest XML

Expand LibvirtConfigGuestCPUTune to allow setting of a
cpu mask per-vCPU

Blueprint: virt-driver-cpu-pinning
Change-Id: I547be6c7da4e3ebc9759f8310cb6282ac4b2f50c
Merge "libvirt: add support for per-vCPU pinning in guest XML"