throw exception when iscsi initiator name is empty

Also fix similar issue for setting "Rsyslog address" and "Netconsole
address" with an empty string

Change-Id: If5e4ac9dd6aa2cc0924281e018b209078f6aba3e
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=981560
Signed-off-by: hadong <hadong0720@gmail.com>