vfio-mediated-device.txt: standardize document format

Each text file under Documentation follows a different
format. Some doesn't even have titles!

In this specific document, the title, copyright and authorship
are added as if it were a C file!

Change its representation to follow the adopted standard,
using ReST markups for it to be parseable by Sphinx:
- convert document preambule to the proper format;
- mark literal blocks;
- adjust identation;
- use numbered lists for references.

Reviewed by: Kirti Wankhede <kwankhede@nvidia.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>