Use smbios to pass systemd credentials instead of fw_cfg

[PhotonQuantum]

According to
systemd/systemd#24204
https://github.com/systemd/systemd/blob/78208556b281bd6ee8a95d16a517152728010475/docs/CREDENTIALS.md?plain=1#L303

fw_cfg has a 55 bytes name length limit, and many systemd creds names are actually longer than that, e.g. opt/io.systemd.credentials/ssh.ephemeral-authorized_keys-all. SMBIOS is the new recommended way to pass in credentials.

[GatienDoesStuff]

Tried looking into this, but it seems like QEMU expects key's values to be embedded within the CLI args, making everything readable from /proc//cmdline on the host :(

This also means a wrapper script would be necessary right ? To read the currently configured secret files into args for QEMU, rather than shove a static store value in ExecStart