Exclude keys from grab

This bug has been copied automatically from: https://bugs.launchpad.net/qemu/+bug/1634069
Reported by 'Konstantin' on 2016-10-17 :

Feature request: pressing every time a shortcut to release grab for
switching windows/desktops is pretty annoying, especially for users of
tiling WMs.

QEMU have to have a way to specify keys or shortcuts (possibly something
like "everything with the specified modifier key"), which would not be
intercepted.

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