storage: Improve error message when a storage backend is missing

Include the name of the storage backend in the error message instead of
just the number.

src/storage/storage_backend.c

@@ -1146,7 +1146,8 @@ virStorageBackendForType(int type)
             return backends[i];
 
     virReportError(VIR_ERR_INTERNAL_ERROR,
-                   _("missing backend for pool type %d"), type);
+                   _("missing backend for pool type %d (%s)"),
+                   type, NULLSTR(virStoragePoolTypeToString(type)));
     return NULL;
 }