libxl: User defined max_maptrack_frames in a stub domain

Implementing qrexec+usbip+qemu in Linux-based stub domain leads me to
an issue where a device model stub domain doesn't have maptrack entries.

Signed-off-by: Dmitry Fedorov <d.fedorov@tabit.pro>
Acked-by: Wei Liu <wl@xen.org>

tools/libs/light/libxl_dm.c

@@ -2311,7 +2311,7 @@ void libxl__spawn_stub_dm(libxl__egc *egc, libxl__stub_dm_spawn_state *sdss)
     dm_config->b_info.target_memkb = dm_config->b_info.max_memkb;
 
     dm_config->b_info.max_grant_frames = guest_config->b_info.max_grant_frames;
-    dm_config->b_info.max_maptrack_frames = 0;
+    dm_config->b_info.max_maptrack_frames = guest_config->b_info.max_maptrack_frames;
 
     dm_config->b_info.u.pv.features = "";