mus: Software and hardware cursor overlap in unified mode.

(1) Run ToT#530902 chrome os on linux desktop with --mus --ash-host-window-bounds=400x400,450+0-800x800*2 --ash-enable-unified-desktop 
(2) Move the mouse around [quickly], while hovering or while dragging a tab/window.
Expected: A single cursor is shown
Actual: Two overlapping cursors are shown

Moving between the two displays, sometimes the cursor scaling looks wrong:
ie. the 'leading' (hardware?) cursor is large/doubled on the 400x400 window.
The fix is to probably ensure we are disabling the hardware cursor and only using the software cursor in unified mode.
This defect was split out from  Issue 804460 .