manual: escape backslashes in "parsed-literal" blocks

manual: escape backslashes in "parsed-literal" blocks

According to
<https://docutils.sourceforge.io/docs/ref/rst/directives.html#parsed-literal>,
"inline markup is recognized and there is no protection from parsing.
Backslash-escapes may be necessary to prevent unintended parsing".

The qemu(1) manual page (formatted with Sphinx 2.2.2) has several overlong
lines on my system. A stand-alone backslash at EOL serves as line
continuation in a "parsed-literal" block. Therefore, escape the
backslashes that we want to appear as such in the formatted documentation.

Cc: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20200908172111.19072-1-lersek@redhat.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>