gtk: Don't warp absolute pointer

This matches the behavior of SDL, and makes the mouse usable when
using -display gtk -vga qxl

https://bugzilla.redhat.com/show_bug.cgi?id=1051724
Signed-off-by: Cole Robinson <crobinso@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
