Don't destroy the vagrant network unless configuration has changed

I'm not 100% happy with this pull request, but I can't think of a better way of doing this. I'm putting this here in the hope that someone will have a clue on how to fix this properly. 

The problem is that if you start multiple vms with vagrant-kvm, this plugin will destroy the existing network config. This causes the previously created vms to loose network connectivity. 

This change only destroys the network config if it has changed. Otherwise, it simply reuses it. This works and solves my issue, but one weird thing about it is that the <host> tag aren't updated. As far as I can see, this does not break anything, but I might be wrong. 

Thoughts?