Check if address is IPv4 and only then add it

This fixes problem when ipset restore failed because IPv6 address was attempted to be added to IPv4 ipset. This commonly resulted in to empty set on initialization (initial synchronizatio).

This is just workaround. Some addresses can be converted from IPv6 to IPv4 and IPv6 ipsets support should be added as well.

Fixes #10 and #9 (closed)