storage/nfs: use string::append() instead of string::insert()

Swap the order of adding the URI and the slash, because appending is
cheaper than inserting.

src/storage/plugins/NfsStorage.cxx

@@ -82,10 +82,9 @@ public:
 static std::string
 UriToNfsPath(const char *uri_utf8)
 {
-	std::string path(uri_utf8);
-
 	/* libnfs paths must begin with a slash */
-	path.insert(path.begin(), '/');
+	std::string path("/");
+	path.append(uri_utf8);
 
 	return path;
 }