net/filter: Fix the output information for command 'info network'

net/filter: Fix the output information for command 'info network'

The properties of netfilter object could be changed by 'qom-set'
command, but the output of 'info network' command is not updated,
because it got the old information through nf->info_str, it will
not be updated while we change the value of netfilter's property.

Here we split a helper function that could collect the output
information for filter, and also remove the useless member
'info_str' from struct NetFilterState.

Signed-off-by: zhanghailiang <zhang.zhanghailiang@huawei.com>
Cc: Jason Wang <jasowang@redhat.com>
Cc: Eric Blake <eblake@redhat.com>
Cc: Markus Armbruster <armbru@redhat.com>
Cc: Yang Hongyang <hongyang.yang@easystack.cn>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>