add default virtcon initialization

When going through the default devices, we don't initialize the virtio
console, unless we're doing -nographic.

I suppose that's just a leftover from the recent code restructuring, so
let's put it in.

Signed-off-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
