From `man xtrabackup`:
```
   The --databases and --databases-file options
       The ` --databases` option accepts a space-separated list of the databases and tables to backup in the databasename[.tablename] format. In addition to this list, make sure to specify the mysql, sys, and performance_schema databases. These databases are required when restoring the databases using xtrabackup --copy-back.

       NOTE:
          Tables  processed  during  the  --prepare  step  may  also  be added to the backup even if they are not explicitly listed by the parameter if they were created after the backup started.

          $ xtrabackup --databases='mysql sys performance_schema test ...'
```