Migration fails when CONF.my_ip is 127.0.0.1

The nova_powervm driver passes the tgt_mgmt_svr parameter to the pypowervm migrate_lpar call as the CONF.my_ip value. In the event this value is '127.0.0.1' the migration will fail in a way that is not clear and inconsistent. The CONF.my_ip value comes from oslo_utils.netutils.get_my_ip4, if for some reason this returns the localhost address, it should be made more clear that migration will fail.

Other drivers may have this same issue.