qapi: handle visitor->type_size() in QapiDeallocVisitor

visit_type_size() requires either visitor->type_size() or
visitor_uint64() to be implemented, otherwise a NULL function pointer is
invoked.

It is possible to trigger this crash as follows:

  $ qemu-system-x86_64 -netdev tap,sndbuf=0,id=netdev0 \
                       -device virtio-blk-pci,netdev=netdev0

The 'sndbuf' option has type "size".

Reviewed-by: Andreas Färber <afaerber@suse.de>
Reviewed-by: Michael Roth <mdroth@linux.vnet.ibm.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>

qapi/qapi-dealloc-visitor.c

@@ -132,6 +132,11 @@ static void qapi_dealloc_type_number(Visitor *v, double *obj, const char *name,
 {
 }
 
+static void qapi_dealloc_type_size(Visitor *v, size_t *obj, const char *name,
+                                   Error **errp)
+{
+}
+
 static void qapi_dealloc_type_enum(Visitor *v, int *obj, const char *strings[],
                                    const char *kind, const char *name,
                                    Error **errp)
@@ -164,6 +169,7 @@ QapiDeallocVisitor *qapi_dealloc_visitor_new(void)
     v->visitor.type_bool = qapi_dealloc_type_bool;
     v->visitor.type_str = qapi_dealloc_type_str;
     v->visitor.type_number = qapi_dealloc_type_number;
+    v->visitor.type_size = qapi_dealloc_type_size;
 
     QTAILQ_INIT(&v->stack);