userportal, webadmin: Show warning for non-exportable disks

Display warning in Create VM template, Export VM, Create VM snapshot dialogs, if non-exportable disks are detected
(e.g. shared disk, direct lun , ...), that these disks will not be part of the export.

For the detection, the Disk.isAllowSnapshot() is used.

Change-Id: I6a183931e36ccd4668225ad206edc4a5e58795e3
Bug-Url: https://bugzilla.redhat.com/855625
Signed-off-by: Libor Spevak <lspevak@redhat.com>