webadmin: tool-tip for DirectLUN disk column

Added a tool-tip for DirectLUN disk column (for each item).
Affected tabs: Disks main-tab and VM -> Disks sub-tab ('All' view).

Change-Id: I939fb57ca024baba86090b9b1840b62a1425dc66
Signed-off-by: Daniel Erez <derez@redhat.com>