Clang warned about a dead assignment.  In the process, I noticed
that we are only using the function for a bool value.  I audited
all other callers in qemu_{migration,cgroup,driver,hotplug), and
all were making the call in a bool context.

Also, do bounds checking on the argument.

* src/qemu/qemu_cgroup.c (qemuSetupCgroup): Delete dead
assignment.
(qemuCgroupControllerActive): Change return type to bool.
* src/qemu/qemu_cgroup.h (qemuCgroupControllerActive): Likewise.