build: silence false positive clang report

build: silence false positive clang report

clang complained that STREQ(group->controllers[i].mountPoint,...)  was
a NULL dereference when i==VIR_CGROUP_CONTROLLER_CPUSET, because it
assumes the worst about virCgroupPathOfController.  Marking the
argument const doesn't yet have an effect, per this clang bug:
http://llvm.org/bugs/show_bug.cgi?id=7758

So, we use sa_assert, which was designed to shut up false positives
from tools like clang.

* src/util/cgroup.c (virCgroupMakeGroup): Teach clang that there
is no NULL dereference.