The xmlto validation produced a number of errors that are now fixed.
Sadly, the DocBook/Makefile still adds --skip-validation to xmlto, so
these errors are missed during a normal compile.

Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>