vm: as SSH key to connect to guest, use ed25519 type instead of rsa
By default, recent SSH (since v8.6) refuses to connect to not-so-old server (e.g. the version on Debian 8) with RSA key.
By default, recent SSH (since v8.6) refuses to connect to not-so-old server (e.g. the version on Debian 8) with RSA key.