[client] spice: use xlib directly for warping

This change allows us to look for and filter out the warp completion
event as we can obtain and use the serial number of the warp request to
do so. This is far more elegant then the x/y match that we were doing
prior.

client/src/main.h

@@ -209,6 +209,7 @@ struct CursorState
 
   /* the mouse warp state and target */
   enum WarpState warpState;
+  unsigned long warpSerial;
   SDL_Point warpTo;
 
   /* the guest's cursor position */