Given that we were overriding qemu_test to enable this anyway,
enabling this by default saves Hydra a QEMU build.

There's also clear demand from users[1] for this feature, so our
alternatives are:

 - Offer a qemu-canokey attribute.  I don't want to do this, because I
   don't think there's any reason to make Hydra build an extra QEMU.

 - Enable it only for qemu_test.  I don't want to do this, because it
   will lead to users using qemu_test without understanding its
   subtleties.

 - Force users to build from source.  I don't think there's any reason
   to do this when it's unlikely to hurt anybody having it enabled by
   default.  There's no reason to single out canokey to be disabled by
   default in spite of users' needs given that we enable so many other
   optional QEMU features.

[1]: canokeys/canokey-qemu#6