make target_args() protected instead of private to allow subclasses to inject args

fesvr/htif.h

@@ -63,6 +63,7 @@ class htif_t : public chunked_memif_t
   virtual void idle() {}
 
   const std::vector<std::string>& host_args() { return hargs; }
+  std::vector<std::string>& target_args() { return targs; }
 
   reg_t get_entry_point() { return entry; }
 
@@ -98,8 +99,6 @@ class htif_t : public chunked_memif_t
   std::vector<device_t*> dynamic_devices;
   std::vector<std::string> payloads;
 
-  const std::vector<std::string>& target_args() { return targs; }
-
   std::map<uint64_t, std::string> addr2symbol;
 
   friend class memif_t;