webadmin: grayed-out LUNs reasons - modify check

Verify that the selected LUN is not apart of a DirectLUN disk first;
since a LUN for DirectLUN disk is grayed-out only if it's already
apart of a Disk in the system.

Change-Id: Ie5d044ed4a75182954bd4a01b5cb071f24da6bd1
Signed-off-by: Daniel Erez <derez@redhat.com>