docs/vm: hwpoison.rst: Fix quote formatting

The asterisks prepended to the quoted text currently get translated to
bullet points, which gets increasingly confusing the smaller your
screen is (when viewing the sphinx output, that is).

Convert the whole quote to a literal block.

Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>