disable devices on capability changes

[Carlos Garnacho]

The first patch had a c/p typo, plus didn't cope too well with the capabilities
being toggled one by one as weston does.

I also noticed that there is another patch around by Boyan Ding to fix the
same issue. It seemed to me that the behavior I implemented is the closest
to what (rootful?) Xorg does. That said, I'm happy with either patch applied,
and the crash gone :).

Cheers,
  Carlos