convert maxfiles to prune_backups when reading the storage configuration

convert maxfiles to prune_backups when reading the storage configuration

If there are already prune options configured, simply delete the maxfiles
setting. Having set both is invalid from vzdump's perspective anyways, and any
backup job on such a storage failed, meaning a user would've noticed.

If there are no prune options, translate the maxfiles value to keep-last,
except for maxfiles being zero (=unlimited), in which case we use keep-all.

If both are not set, don't set anything, so:
1. Storages don't suddenly have retention options set.
2. People relying on vzdump defaults can still use those.

Signed-off-by: Fabian Ebner <f.ebner@proxmox.com>