Add a feature for lxc export so that it leaves the export on the server

[aivanise]

as discussed in discourse, can you please add an option for lxc export to leave the export on the server once it is done and not try to stream it to the user, something like

lxc export myct filename --leave-on-server that would leave the backup in /var/snap/lxd/common/lxd/backups/filename or on the backup storage volume if configured

ideally it would also be an option to fire and forget (--nowait?) or it can be the default, since in this case it makes no sense to wait for nothing, but it might be useful to know that the backup is done once lxc export exits.

[edlerd]

Thank you for the request.

The command line lxc export is meant to get an instance out of the server, so we won't add support for keeping the export on the server.

With that in mind, I see that the API has an option to set an expiration time on the POST /1.0/instances/:name/backups endpoint. You could use it to create an export with a very long expiration date, basically keeping it on the server.