In fullscreen mode, the window property of cocoaView may not be the key
window, and the current implementation would not re-grab cursor by left click
in fullscreen mode after ungrabbed in fullscreen mode with hot-key ctrl-opt-g.

This patch used value of isFullscreen as a short-cirtuit condition for
relative input device grabbing.

Signed-off-by: Chen Zhang <tgfbeta@me.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 2D2F1191-E82F-4B54-A6E7-73FFB953DE93@me.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>