How do I calculate %CPU in my own libvirt programs?

Virt-top FAQ ( http://people.redhat.com/~rjones/virt-top/faq.html#calccpu )

Simple %CPU usage for a domain is calculated by sampling virDomainGetInfo periodically and looking at the virDomainInfo cpuTime field. This 64 bit field counts nanoseconds of CPU time used by the domain since the domain booted.
Let t be the number of seconds between samples. (Make sure that t is measured as accurately as possible, using something like gettimeofday(2) to measure the real sampling interval).
Let cpu_time_diff be the change in cpuTime over this time, which is the number of nanoseconds of CPU time used by the domain, ie:

cpu_time_diff = cpuTimenow — cpuTimet seconds ago

Let nr_cores be the number of processors (cores) on the system. Use virNodeGetInfo to get this.
Then, %CPU used by the domain is:

%CPU = 100 × cpu_time_diff / (t × nr_cores × 109)

Because sampling doesn't happen instantaneously, this can be greater than 100%. This is particularly a problem where you have many domains and you have to make a virDomainGetInfo call for each one.