iscsi: detecting empty LUN list

Detecting and reporting an empty LUN list to provide a better user experience.

Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1133937

Change-Id: Iab293c172746a36779f8c2623c1c2542bf5aaca0
Signed-off-by: Simone Tiraboschi <stirabos@redhat.com>