Mysql database design with XML
If the only thing you are storing in the database is the XML data, what is to stop you from storing the file directly on the filesystem and bypassing the overhead of querying the database for the single field?

If backups are your concern, there are methods to backup a directory as well.

Usage of MySQL (or other RDBMS) implies there is a relational aspect to the data you are storing. If there's no relationship, this is probably not the tool you are looking for.