webadmin: Show warning, if VM provides any non-exportable disks

The test is performed for image non-sharable disks and direct LUNs before
creating VM export, new VM template or VM snapshot.

All messages ure unified to 3 similar forms (0-3 of them can be displayed based on the
condition)

Change-Id: I339da4bcab18799603caeafb0171fdcea262ec29
Bug-Url: https://bugzilla.redhat.com/855625
Signed-off-by: Libor Spevak <lspevak@redhat.com>