virCgroupController: Check the enum fits into 'int'

virCgroupController: Check the enum fits into 'int'

Throughout our code, the virCgroupController enum is used in two ways.
First as an index to an array of cgroup controllers:

struct virCgroup {
    char *path;

    struct virCgroupController controllers[VIR_CGROUP_CONTROLLER_LAST];
};

Second way is that when calling virCgroupNew() a bitmask of the enum
items can be passed to selectively detect only some controllers. For
instance:

int
virCgroupNewVcpu(virCgroupPtr domain,
                 int vcpuid,
                 bool create,
                 virCgroupPtr *group)
{
    ...
    controllers = ((1 << VIR_CGROUP_CONTROLLER_CPU) |
                   (1 << VIR_CGROUP_CONTROLLER_CPUACCT) |
                   (1 << VIR_CGROUP_CONTROLLER_CPUSET));

    if (virCgroupNew(-1, name, domain, controllers, group) < 0)
        goto cleanup;
}

Even though it's highly unlikely that so many new controllers will be
invented so that we would overflow when constructing the bitmask, it
doesn't hurt to check at compile time either.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>