frontend: disable stateless mode for diskless vm

This patch disable the 'Run Stateless' check-box in the run-once dialog,
in both webadmin and user portal, for diskless VM.

Change-Id: I54a3c720edf863dcbc520ef41194d3816f9a0048
Bug-Url: https://bugzilla.redhat.com/907968
Signed-off-by: Arik Hadas <ahadas@redhat.com>