remus: libcheckpoint - initialize unused callback fields to NULL

Add a memset to the save_callbacks struct instance in libcheckpoint's
initialization code. New additions to the callback struct will not
need to add an explicit initialization (to NULL), to maintain
compatibility with older xend/remus based invocation of xc_domain_save.

Signed-off-by: Shriram Rajagopalan <rshriram@cs.ubc.ca>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Committed-by: Ian Jackson <ian.jackson@eu.citrix.com>

tools/python/xen/lowlevel/checkpoint/checkpoint.c

@@ -155,6 +155,7 @@ static PyObject* pycheckpoint_start(PyObject* obj, PyObject* args) {
   } else
     self->checkpoint_cb = NULL;
 
+  memset(&callbacks, 0, sizeof(callbacks));
   callbacks.suspend = suspend_trampoline;
   callbacks.postcopy = postcopy_trampoline;
   callbacks.checkpoint = checkpoint_trampoline;