spice: add tablet support

Add support for the spice tablet interface.  The tablet interface will
be registered (and then used by the spice client) as soon as a absolute
pointing device is available and used by the guest, i.e. you'll have to
configure your guest with '-usbdevice tablet'.
diff --git a/ui/spice-display.c b/ui/spice-display.c
index 0bc230e..6702dfd 100644
--- a/ui/spice-display.c
+++ b/ui/spice-display.c
@@ -180,7 +180,7 @@
     surface.width      = ds_get_width(ssd->ds);
     surface.height     = ds_get_height(ssd->ds);
     surface.stride     = -surface.width * 4;
-    surface.mouse_mode = 0;
+    surface.mouse_mode = true;
     surface.flags      = 0;
     surface.type       = 0;
     surface.mem        = (intptr_t)ssd->buf;