libvirt pmda: add new block, perf, vcpu metrics
libvirt pmda: add new block, perf, vcpu metrics

For an overview, see

https://groups.io/g/pcp/message/15733

For more details on the new perf metrics, see

https://libvirt.org/formatdomain.html

For explanation on some of the block metrics, see

https://www.berrange.com/posts/2017/02/10/the-surprisingly-complicated-world-of-disk-image-sizes/

adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp