util: cgroup: Use GHashTable instead of virHashTable

util: cgroup: Use GHashTable instead of virHashTable

Rewrite using GHashTable which already has interfaces for using a number
as hash key. Glib's implementation doesn't copy the key by default, so
we need to allocate it, but overal the interface is more suited for this
case.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Pavel Hrdina <phrdina@redhat.com>