nwfilter_define: Update case under update_exist_filter

Since bug 1077009 fixed, the logic updated to not allow update
filter with same name after 1.2.7, so decide the status_error
according to libvirt version for work around this.

Signed-off-by: Wayne Sun <gsun@redhat.com>
Merge pull request #382 from waynesun09/nwfilter_bug1077009

nwfilter_define: Update case under update_exist_filter