Fix pointer position handling

This change fixes the handling of the pointer position coordinates for pointers whose hotspots are not at the upper left-hand corner. Before this change, any pointer whose hotspot was not in the upper left-hand corner was displayed in the wrong position by spice-gtk when the guest was using this driver with the QEMU QXL device. The issue was especially obvious in applications like Microsoft Paint, which uses a cross-hair pointer whose center is supposed to be displayed at the cursor position.

I'm a little bit surprised that nobody else has reported this issue, but I've been using my fix for two months, and all the cursors have been in the correct position with the updated driver. This is my first foray into the SPICE/QXL code and Windows device drivers, so please let me know if I'm mistaken about any of this.