util: Adjust qemu_guest_getrandom_nofail for Coverity

Explicitly ignore the return value of qemu_guest_getrandom.
Because we use error_fatal, all errors are already caught.

Fixes: CID 1401701
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20190530173824.30699-1-richard.henderson@linaro.org>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>