conf: never reject <disk> <seclabel relabel='no'> overrides

conf: never reject <disk> <seclabel relabel='no'> overrides

Trying to disable <seclabel> for the whole <domain> and _also_
disable <seclabel> at the <disk> level will fail with:

  error: unsupported configuration: label overrides require relabeling to be enabled at the domain level

which seems wrong. Instead skip the validation when disk seclabel
has relabel='no', that config should always be valid.

Reviewed-by: Michal Privoznik <mprivozn@redhat.com>
Signed-off-by: Cole Robinson <crobinso@redhat.com>