Test if the device is grabbed in some of the evdev plugins

Test if the device is grabbed in some of the evdev plugins

Even though they target embedded devices, the touch and tablet plugins
are very often used on desktop, under X, for development and
experimenting. However the X.org drivers like to grab the input
devices, resulting in getting no events when reading
directly. Inspired by evtest, we show a warning from now on in this
case so the user has a clue what's going on.

Change-Id: I19474f1fcc5163f2601e7000aaee0cce54f99367
Reviewed-by: Samuel Rødal <samuel.rodal@nokia.com>