add new API virDomainGetCPUStats() for getting cpu accounting information
per real cpus which is used by a domain.  The API is designed to allow
future extensions for additional statistics.

based on ideas by Lai Jiangshan and Eric Blake.

* src/libvirt_public.syms: add API for LIBVIRT_0.9.10
* src/libvirt.c: define virDomainGetCPUStats()
* include/libvirt/libvirt.h.in: add virDomainGetCPUStats() header
* src/driver.h: add driver API
* python/generator.py: add python API (as not implemented)
Signed-off-by: NEric Blake <eblake@redhat.com>