ipvs: register hooks only with services

Keep the IPVS hooks registered in Netfilter only
while there are configured virtual services. This
saves CPU cycles while IPVS is loaded but not used.

Signed-off-by: Julian Anastasov <ja@ssi.bg>
Reviewed-by: Simon Horman <horms@verge.net.au>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>