Watch for dynamips rom & nvram change

This monitor for change the file from dynamips by computing a
hash of the watched file.

The way dynamips work prevent the update of the modification time.

We can improve that by using native system for watching file but:
* it's require dependencies specific for each OS
* dependencies use C extensions
* this is only a backup if your router is cleanly shutdown we export
stuff

gns3server/compute/iou/iou_vm.py

@@ -503,7 +503,7 @@ class IOUVM(BaseNode):
             # check if there is enough RAM to run
             self.check_available_ram(self.ram)
 
-            self._nvram_watcher = FileWatcher(self._nvram_file(), self._nvram_changed)
+            self._nvram_watcher = FileWatcher(self._nvram_file(), self._nvram_changed, delay=10)
 
             # created a environment variable pointing to the iourc file.
             env = os.environ.copy()