The SDBus will reuse these functions, so we put them in a new source file.

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-id: 20180309153654.13518-3-f4bug@amsat.org
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
[PMM: slight wordsmithing of comments, added note that string
 returned does not need to be freed]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>