memtohex() adds an extra trailing NUL character.

Reported-by: AddressSanitizer
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-id: 20180408145933.1149-1-f4bug@amsat.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>