Feature: libpe_status: Drop support for multiple top-level nvset rules

Multiple top-level rules in an nvset (cluster_property_set,
instance_attributes, meta_attributes, utilization) were already
disallowed by the schema, so it was only possible to configure them with
schema validation disabled.

Use nested rules instead.

Signed-off-by: Reid Wahl <nrwahl@protonmail.com>