conf: don't use passed in caps in post parse method

conf: don't use passed in caps in post parse method

To enable the virCapsPtr parameter to the post parse method to be
eliminated, the drivers must fetch the virCapsPtr from their own
driver via the opaque parameter, or use an alternative approach
to validate the parsed data.

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