Isolate guests from host networking

Uses VB to proxy DNS requests in order to allow the host to move between
different networks.

Tested on Mint 13 / Ubuntu 12.04 with new resolvconf and dnsmasq setup.
Needs testing on other platforms and investigate whether there is any
performance hit, before merging up.
Merge branch 'master' into dns_proxy
Merge branch 'master' into dns_proxy
Merge branch 'dns_proxy'