perf: add new public APIs for perf event

perf: add new public APIs for perf event

API agreed on in
https://www.redhat.com/archives/libvir-list/2015-October/msg00872.html

* include/libvirt/libvirt-domain.h (virDomainGetPerfEvents,
virDomainSetPerfEvents): New declarations.
* src/libvirt_public.syms: Export new symbols.
* src/driver-hypervisor.h (virDrvDomainGetPerfEvents,
virDrvDomainSetPerfEvents): New typedefs.
* src/libvirt-domain.c: Implement virDomainGetPerfEvents and
virDomainSetPerfEvents.

Signed-off-by: Qiaowei Ren <qiaowei.ren@intel.com>
Message-id: 1459171833-26416-2-git-send-email-qiaowei.ren@intel.com