sysprep: remove firewall rules

If we do not want to keep the original firewall rules, enable
this operation to remove these rules automatically.

Signed-off-by: Wanlong Gao <gaowanlong@cn.fujitsu.com>

RWMJ:

 - Updated po-file.

 - Change documentation to note that it's disabled by default
   and in theory could make the guest exploitable.