vzdump: defaults: convert to prune-backups early enough

vzdump: defaults: convert to prune-backups early enough

Fixes the case where reading from /etc/vzdump.conf fails.

Also convert the options read from /etc/vzdump.conf before the loop. That
avoids showing a wrong warning when 'prune-backups' is configured in
/etc/vzdump.conf, and maxfiles isn't. Previously, because 'maxfiles' from the
schema defaults was automatically set, the call to parse_prune_backups_maxfiles
after the loop threw the warning that both options are defined.

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