Fix multiple formatting problems in HTML docs

Fix multiple formatting problems in HTML docs

The rule generating the HTML docs passing the --html flag
to xsltproc. This makes it use the legacy HTML parser, which
either ignores or tries to fix all sorts of broken XML tags.
There's no reason why we should be writing broken XML in
the first place, so removing --html and adding the XHTML
doctype to all files forces us to create good XML.

This adds the XHTML doc type and fixes many, many XML tag
problems it exposes.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>