Add RelaxNG schemas for the XML files

Also hook them up to the 'check' target in the Makefile.

Signed-off-by: Vratislav Podzimek <vpodzime@redhat.com>
Install the RelaxNG schema files

- Add the validation to the %check section of the .spec file
- Add a Makefile target to conveniently create the .rnc files
  for editing with Emacs