Fix mouse moves upscaling

MOUSE_REL_MULT_DIVIDEND (30000) and REL_MULT (50) seem to be approximates for up-scaling the mouse moves to the size of the absolute input device that gets exposed to the guests.

REL_MULT_DIVIDENT was used as the absolute input size, instead of MAX_MOUSE_ABS_[X,Y]
REL_MULT was used as a multiplier, instead of [size of the absolute mouse] divided by [resolution]

I tested that fix, along with a workaround for win-tools (see https://github.com/OpenXT/win-tools/pull/1), and got very smooth mouse movements in a 4-monitors-wide configuration.

The only thing I'm not completely sure about is the MOUSE_DIV_THRESHOLD thing for mouse speed. I can remove that part of the commit if requested.