libvirt: introduce separate class for cpu tune XML config

Instead of just setting attributes against the main
LibvirtConfigGuest object, have a LibvirtConfigGuestCPUTune
object. This is more flexible when we have to add support
for further cpu tuning attributes.

Blueprint: virt-driver-cpu-pinning
Change-Id: Id3667752fcbbfe3e55ee221fd2cd364d5cfb2f80
Merge "libvirt: introduce separate class for cpu tune XML config"