vt(4): Fix mouse cursor handling in vt_fb/creator_vt/ofwfb

There were two issues:
    1. The area given to vt_is_cursor_in_area() was adding the drawable
       area offset, something already handled by this function.
    2. The cursor was shifted on the screen by the offset of this area
       and thus was misplaced or not erased. Furthermore, when reaching
       the bottom or right borders, the cursor was either totally
       removed or not erased correctly.

MFC after:	1 week