QEMU USB Tablet is not really a tablet - it's a trick

Unfortunately, Qemu gives us this confusing device to work with,
and the best we can do is filter it out based on its name.

https://bugzilla.gnome.org/show_bug.cgi?id=760445

gdk/x11/gdkdevicemanager-xi2.c

@@ -428,6 +428,7 @@ create_device (GdkDeviceManager *device_manager,
         input_source = GDK_SOURCE_PEN;
       else if (!strstr (tmp_name, "mouse") &&
                !strstr (tmp_name, "pointer") &&
+               !strstr (tmp_name, "qemu usb tablet") &&
                has_abs_axes (display, dev->classes, dev->num_classes))
         input_source = GDK_SOURCE_TOUCHSCREEN;
       else