vzdump: set task mode early to fix a warning

vzdump: set task mode early to fix a warning

when a vzdump script is set and the backup fails early (eg.
when exceeding the number of backups) run_hook_script()
showed an uninitialized value error trying to use
$task->{mode} which is set only after prepare() was called.

This sets $task->{mode} early, still updated later to $stop
if !$running, and changes the condition for whether
cleanup() should be called to not use $task->{mode} (which
makes no real sense anyway) to using the $cleanup hash like
the rest of the code.