core: trust SMBIOS credentials under Intel TDX

core: trust SMBIOS credentials under Intel TDX

SMBIOS OEM strings are host-controlled and normally distrusted by PID1
in confidential VMs. Under TDX, however, the SMBIOS table (including
Type 11) is measured into RTMR0 by the firmware (TDVF), so a remote
verifier can detect host tampering with credentials delivered this
way. Accept them there, while keeping fw_cfg distrusted as those
items are not measured even on TDX.

This lets systemd-vmspawn deliver credentials to TDX guests via the
normal SMBIOS path, unlike SNP which requires the initrd cpio channel.

Signed-off-by: Paul Meyer <katexochen0@gmail.com>