man: document SEV-SNP credential delivery via initrd cpio
Under --coco=sev-snp, credentials no longer flow through SMBIOS/fw_cfg
(which the guest PID1 discards as unmeasured in confidential VMs) but
through a cpio archive appended to the initrd, landing in the @system
bucket via the new /.extra/system_credentials/ initrd path. Update
systemd-vmspawn(1) to describe this and the guest systemd version
requirement.