vircgroup: Ensure /machine group is associated with its parent

vircgroup: Ensure /machine group is associated with its parent

Call first virCgroupNew on the parent group virCgroupNewPartition if
it is available on before the creation of the child group.  This
ensures that the creation of a first level group on the unified
architecture, as the check at virCgroupV2ParseControllersFile as the
parent file is there.

Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=1760233
Signed-off-by: Miguel Ángel Arruga Vivas <rosen644835@gmail.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>