libvirt: convert cpuset XML handling to use set instead of string

The libvirt guest XML config class allowed the cpuset to be
specified as a string. Convert this to use a set() and the
standard CPU set parsing/formatting routines

Blueprint: virt-driver-numa-placement
Blueprint: virt-driver-cpu-pinning
Change-Id: Ice69de4dd6ce0b52b0eb7b4cbdc4e28d1111f4bf
Merge "libvirt: convert cpuset XML handling to use set instead of string"