docs: clean-up the xenpvh documentation

docs: clean-up the xenpvh documentation

I noticed the code blocks where not rendering properly so thought I'd
better fix things up. So:

  - Use better title for the machine type
  - Explain why Xen is a little different
  - Add a proper anchor to the tpm-device link
  - add newline so code block properly renders
  - add some indentation to make continuation clearer

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-ID: <20231207130623.360473-1-alex.bennee@linaro.org>