This is especially useful for proxy VMs that e.g. run some transparent proxy service such as tor,
and need to rebind it upon IP change (of course this assumes iptables-based transparent redirection
such as DNAT).
Apparently vif frontend has broken sg implementation; we already worked around
it in init.d script via ethtool; now do the same in setup_ip. It is relevant
when attaching firewallvm to a different netvm on the fly.
