nbd/server: Support inactive nodes

nbd/server: Support inactive nodes

In order to support running an NBD export on inactive nodes, we must
make sure to return errors for any operations that aren't allowed on
inactive nodes. Reads are the only operation we know we need for
inactive images, so to err on the side of caution, return errors for
everything else, even if some operations could possibly be okay.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Acked-by: Fabiano Rosas <farosas@suse.de>
Message-ID: <20250204211407.381505-14-kwolf@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>