Build the zero page inside stage0

[andrisaar]

The key thing in the zero page is the E820 memory map of what physical memory address ranges are valid.

QEMU provides the fw_cfg API for this: https://github.com/qemu/qemu/blob/master/docs/specs/fw_cfg.rst

Unfortunately the exact method of finding out valid memory areas via fw_cfg is largely undocumented and thus may require some digging into both QEMU and/or SeaBIOS code to figure out how to do properly.