conf: move seclabel validation into post-parse phase

conf: move seclabel validation into post-parse phase

Currently the disk and chardev seclabels are validated immediately at
the time their data is parsed. This forces the parser to fill in the
top level secmodel at time of parsing which is an undesirable thing.
This validation conceptually should be done in the post-parse phase
instead.

Reviewed-by: Michal Privoznik <mprivozn@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>