audio: catch missing sdl support

sdl is probed before audio, so we can simply look at $sdl so see
whenever we have support or not.  Throw an error in case sdl audio
is requested without sdl being available.

Reported-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Tested-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 1490000743-3615-1-git-send-email-kraxel@redhat.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>