Created attachment 362590 [details, diff] spice-0.12.4-alsa-conditional.patch See also http://cgit.freedesktop.org/spice/spice/commit/?id=58195c195110f0dbb24d831b5eefb2f094848b26 And the attached ebuild patch.
Nice job. I notice you're the upstream author. Go ahead and commit this to the tree.
Applied. I'm just the author of the patch :D