This change adds the ablity for a user or operator to contol
the virtualisation of a performance monitoring unit within a vm.
This change introduces a new "hw:pmu" extra spec and a corresponding
image metadata property "hw_pmu".
The glance image metadata doc will be updated seperately by:
https://review.opendev.org/#/c/675182
Change-Id: I5576fa2a67d2771614266022428b4a95487ab6d5
Implements: blueprint libvirt-pmu-configuration