engine: Do not add disk in snapshot preview

Do not allow to add image disk, sharable disk or LUN to a VM
in snapshot preview mode (in shutdown or up states).

The condition is indicated by the error dialog message:
"Cannot add Virtual Machine Disk. VM is previewing a Snapshot."

Change-Id: Ibfde0540f1d77456ed1412b015a0963f4682039f
Bug-Url: https://bugzilla.redhat.com/909708
Signed-off-by: Libor Spevak <lspevak@redhat.com>