Fix device creation

Instead of registering a new device everytime the compositor
tells us it has keyboard or mouse capability, register
mouse and keyboard once for all.

Handle better when the server says it doesn't have anymore the mouse
or keyboard capability.

This fixes VT switch handling: after some VT switch,
the Xserver would crash because it would have created
too much devices.

Signed-off-by: Axel Davy <axel.davy@ens.fr>