import/export daemon: Add support for a magic prefix

This “magic” value will be used to ensure that we don't accidentially
connect to the wrong daemon (e.g. due to a bug), comparable to DRBD's
per-disk secret. Just depending on the SSL certificate isn't enough
as it's always per instance and not per disk.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>