hva: fix host_addr detection.

one of Isono::Util.default_gw_ip dependency issue #199 

if host node has no default route, Isono::Util.default_gw_ip cause issue.

```
undefined method `split' for nil:NilClass
        /opt/axsh/wakame-vdc/dcmgr/vendor/bundle/ruby/1.9.1/gems/isono-0.2.18/lib/isono/util.rb:44:in `default_gw_ipaddr'
```