Throw an error if there's another host with the same IP in the subnet.

vzctl displays a warning if there is a host in the same subnet with the same IP that's being assigned to CT. This behaviour leads to DoS of another container/host, which is unacceptable in production environment. Proposed patch causes vzctl to produce an error instead of warning, leaving the container fully alive with no networking, that doesnt cause any impact on already working CT with the same IP.

refs http://bugzilla.openvz.org/show_bug.cgi?id=869