webadmin: No exportable disk warning text fixed

The warning for diskless VM rephrased (for VM export/template/snapshot dialogs).

Change-Id: I4625f756eacc07c6aa58c12ac49b39b431330451
Bug-Url: https://bugzilla.redhat.com/855625
Signed-off-by: Libor Spevak <lspevak@redhat.com>