ossaudio: check for oss support in oss_audio_init

Check whenever the device path (/dev/dsp by default) exists and qemu is
allowed to access it.  Return NULL if it isn't, so ossaudio will not
be used on systems wihtout oss support (increasinly common on modern
linux systems).

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
