In some cases when the SMBIOS tables are small enough (both in legacy and non-legacy mode) - These tables are allocated in FSEG instead of high-mem, specifically Type0->release_date string which might cause SystemBiosDate to change - depending on its value. This leads to an inconsistent behaviour that depends on the SMBIOS table sizes.
We fix this inconsistency by controlling which dates appear in FSEG:
* Introducing fw_cfg key 'etc/win-bios-date' to control the date that appears in FSEG(0xFFF5).
I don't think we need a separate fw_cfg file for this.
If qemu provides a type0 table we can just take the date from it.
If qemu does not provide a type0 table we should not have a inconsistency in the first place.