api.server: Provide the virtual config instead of the config

The checks ensure the virtual config was ok and then went on returning the
initial configuration (which was possibly incomplete).

This commit fixes it.

swh/vault/api/server.py

@@ -84,12 +84,14 @@ def check_config(cfg: Dict[str, Any]) -> Dict[str, Any]:
         vcfg["storage"] = cfg.get("storage")
     if "scheduler" not in vcfg:
         vcfg["scheduler"] = cfg.get("scheduler")
+    if "client_max_size" not in vcfg:
+        vcfg["client_max_size"] = cfg.get("client_max_size")
 
     for key in ("cache", "storage", "scheduler"):
         if not vcfg.get(key):
             raise ValueError(f"invalid configuration: missing {key} config entry.")
 
-    return cfg
+    return vcfg
 
 
 def make_app(config: Dict[str, Any]) -> VaultServerApp: