test-playback: Pass proper types to spice_server_add_interface
test-playback: Pass proper types to spice_server_add_interface

This is a revert of b76e561d.
For a SpicePlaybackInstance, the base interface must be a
SpicePlaybackInterface instance, not a SpiceBaseInterface instance, or
spice-server code will end up reading out of bounds.

Signed-off-by: Christophe Fergeau <05b95054a663c388a4e33d95828cb30f0370f8ef@redhat.com>
Acked-by: Frediano Ziglio <55d48b080b2e443e395cde84d2c83b135a4ff48e@redhat.com>

fgouget/spice,fgouget/spice,fgouget/spice,fgouget/spice