qemu: fix memory leak in virDomainGetVcpus
https://bugzilla.redhat.com/show_bug.cgi?id=808979
The leak is really in virProcessInfoGetAffinity, as shown in the
valgrind output given in the above bug report - it calls CPU_ALLOC(),
but then fails to call CPU_FREE().
This leak has existed in every version of libvirt since 0.7.5.