For migration set the instance's host to dest during iptables setup

The instance's host is temporarily set to the destination when
performing the call to pre_live_migration to setup the iptables
on the remote host. This is required to enusre the correct DHCP
filtering rules are applied.

Change-Id: I25b4df185e2cb9c0fca4e59c4e0f4824842c3667