util: cgroup: modify virCgroupFree to take virCgroupPtr

util: cgroup: modify virCgroupFree to take virCgroupPtr

Modify virCgroupFree function signature to take a value of type
virCgroupPtr instead of virCgroupPtr * as the parameter.

Change the argument type in all calls to virCgroupFree function
from virCgroupPtr * to virCgroupPtr. This is a step towards
having consistent function signatures for Free helpers so that
they can be used with VIR_AUTOPTR cleanup macro.

Signed-off-by: Sukrit Bhatnagar <skrtbhtngr@gmail.com>
Reviewed-by: Erik Skultety <eskultet@redhat.com>