storage: Fix check for stable path check

Fix the if (!STRPREFIX(path, "/dev")) to be if (!STRPREFIX(path, "/dev/"))
to ensure a path such as "/device" isn't declared stable.

src/storage/storage_backend.c

@@ -1680,7 +1680,7 @@ virStorageBackendPoolPathIsStable(const char *path)
     if (path == NULL || STREQ(path, "/dev") || STREQ(path, "/dev/"))
         return false;
 
-    if (!STRPREFIX(path, "/dev"))
+    if (!STRPREFIX(path, "/dev/"))
         return false;
 
     return true;