RFC: Add support for virt machine to Seabios

[Rob Bradford]

The virt machine in the NEMU project has the ambition to be a platform
that has no emulated legacy hardware. This patch series enables support
for that machine type in Seabios.

The impact that this has for a Seabios port is that the CMOS is not
available to query details of CPUs or memory configuration; instead this
patch series modifies the code that queries those details to prefer
those from QEMU FW CFG over CMOS.

I've tested this patch series with pc, Q35 and virt as part of our
automated testing for NEMU.

Rob