cocoa.m: Fix scroll wheel support

When using a mouse's scroll wheel in a guest with
the cocoa front-end, the mouse pointer moves up
and down instead of scrolling the window. This
patch fixes this problem.

Signed-off-by: John Arbuckle <programmingkidx@gmail.com>
Message-id: 20180108180707.7976-1-programmingkidx@gmail.com
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>