Restrict instance move to templates based on local files

Moving an instance is done by copying over the disks. Restrict
this to disk templates that are copyable. This avoids accidental
use on, e.g., the sharedfile template.

Signed-off-by: Klaus Aehlig <aehlig@google.com>
Signed-off-by: Bernardo Dal Seno <bdalseno@google.com>
Reviewed-by: Bernardo Dal Seno <bdalseno@google.com>