Check whenever we actually found the spiceaudio driver
before flipping the can_be_default field.

Fixes: f0c4555e ("audio: remove qemu_spice_audio_init()")
Buglink: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=977301


Reported-by: dann frazier <dann.frazier@canonical.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Message-Id: <20201215081151.20095-1-kraxel@redhat.com>