Checks if IOS image exist at startup and not during node creation. Fixes #240.

gns3server/modules/dynamips/nodes/router.py

@@ -462,9 +462,6 @@ class Router(BaseVM):
 
         image = self.manager.get_abs_image_path(image)
 
-        if not os.path.isfile(image):
-            raise DynamipsError("IOS image '{}' is not accessible".format(image))
-
         yield from self._hypervisor.send('vm set_ios "{name}" "{image}"'.format(name=self._name, image=image))
 
         log.info('Router "{name}" [{id}]: has a new IOS image set: "{image}"'.format(name=self._name,