iscsi: Fix localhost iscsi target login bug

By default, 'hostname' will be used as portal, and it will get timeout
when login this target while host ipv6 enabled:
Logging in to [iface: default, target: XXXX, portal: fe80::baac:6fff:fe3b:9aa,3260] (multiple)
iscsiadm: Could not login to [iface: default, target: XX, portal: fe80::baac:6fff:fe3b:9aa,3260].
iscsiadm: initiator reported error (8 - connection timed out)

So use 'localhost' to discover and login the target will get rid of
this problem.

Signed-off-by: Yanbing Du <ydu@redhat.com>