webadmin: snapshot custom preview - excluded disks warning

Snapshot custom preview dialog:
Display warning when excluding disks from the snapshot
(in order to emphasize that these disks will be removed).

Change-Id: I1fd866d6553d73233f0bb590d47c370d11ddd8f5
Bug-Url: https://bugzilla.redhat.com/1111983
Bug-Url: https://bugzilla.redhat.com/1111984
Signed-off-by: Daniel Erez <derez@redhat.com>