fix(xo-server/backup): pass correct path to isValidXva

The path was incorrect which means there was a warning in xo-server logs and the check was not properly working.

packages/xo-server/src/xo-mixins/backups-ng/index.js

@@ -1449,7 +1449,7 @@ export default class BackupNg {
               )
 
               if (handler._getFilePath !== undefined) {
-                await isValidXva(handler._getFilePath(dataFilename))
+                await isValidXva('/' + handler._getFilePath(dataFilename))
               }
 
               await handler.outputFile(metadataFilename, jsonMetadata)