virt-host-validate: rewrite cgroup detection to use util/vircgroup

virt-host-validate: rewrite cgroup detection to use util/vircgroup

This removes code duplication and simplifies cgroup detection.
As a drawback we will not have separate messages to enable cgroup
controller in kernel or to mount it.  On the other side the rewrite
adds support for cgroup v2.

The kernel config support was wrong because it was parsing
'/proc/self/cgroup' instead of '/proc/cgroups/' file.

The mount suggestion is removed as well because it will not work
with cgroup v2.

Signed-off-by: Pavel Hrdina <phrdina@redhat.com>