virt: add method for formatting CPU sets to strings

Add a format_cpu_spec method to nova.virt.hardware to
act as the inverse to parse_cpu_spec, by converting a
set/list of CPU indexes into a range string.

Blueprint: virt-driver-numa-placement
Blueprint: virt-driver-cpu-pinning
Change-Id: I3d4f38456111dd44f0ce6f9dd9cbcd2a8cf7d485
Merge "virt: add method for formatting CPU sets to strings"