TODO

Optimize Model::get_domUs() in inserting in Model::$domUs_by_names array only
halted domUs.

Make Model::create_user() handles PDOException when a unique constraint is
violated to properly returns false.

Model::get_user()
Model::get_users()
"guest" must always exists, even if he is not in the database.

Model::delete_users()
Model::create_users()
Uses the cache to avoid unnecessary operations.

Model::delete_users()
Model::create_users()
Model::get_user()
Model::get_users()
Provides a "$ignore_cache" parameter.

Handles the "recovery" user (almighty).

SQL
When an user is deleted, also delete his ACLs.


Live migration

Handle more correctly errors sent throught JavaScript.