docs: kvm: Convert s390-diag.txt to ReST format

This file is almost in ReST format. Just one change was
needed:

    - Add markups for a literal block and change its indentation.

While here, use the standard markup for the document title.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>