Disable inter-cluster instance move for file instances

Inter-cluster instance moves depends on exporting an instance. For file-based
instance, exporting is not supported. Therefore, this patch disables
inter-cluster instance moves for file-based instances and informs the user
with an expressive error message.

Signed-off-by: Helga Velroyen <helgav@google.com>
Reviewed-by: Michele Tartara <mtartara@google.com>