If the cursor is hidden (SDL_ShowCursor(0)) and the input is grabbed
(SDL_WM_GrabInput(SDL_GRAB_ON)), then the mouse will give relative motion
events even when the cursor reaches the edge fo the screen. This is currently
only implemented on Windows and Linux/Unix-a-likes.

Index: qemu/sdl.c
===================================================================
--- qemu.orig/sdl.c	2006-04-13 12:22:22.000000000 +0100
+++ qemu/sdl.c	2006-05-25 00:28:25.000000000 +0100
@@ -280,13 +280,13 @@
 
 static void sdl_hide_cursor(void)
 {
-    SDL_SetCursor(sdl_cursor_hidden);
+    SDL_ShowCursor(0);
 }
 
 static void sdl_show_cursor(void)
 {
     if (!kbd_mouse_is_absolute()) {
-	SDL_SetCursor(sdl_cursor_normal);
+        SDL_ShowCursor(1);
     }
 }