pid1: import creds from SMBIOS too, not just qemu's fw_cfg

pid1: import creds from SMBIOS too, not just qemu's fw_cfg

This imports credentials also via SMBIOS' "OEM vendor string" section,
similar to the existing import logic from fw_cfg.

Functionality-wise this is very similar to the existing fw_cfg logic,
both of which are easily settable on the qemu command line.

Pros and cons of each:

SMBIOS OEM vendor strings:
   - pro: fast, because memory mapped
   - pro: somewhat VMM independent, at least in theory
   - pro: qemu upstream sees this as the future
   - pro: no additional kernel module needed
   - con: strings only, thus binary data is base64 encoded

fw_cfg:
   - pro: has been supported for longer in qemu
   - pro: supports binary data
   - con: slow, because IO port based
   - con: only qemu
   - con: requires qemu_fw_cfg.ko kernel module
   - con: qemu upstream sees this as legacy