vmd: create a copy of bios at 4g boundary

vmd: create a copy of bios at 4g boundary

Newer Linux kernels call into the bios to perform a reboot and our
version of SeaBIOS assumes there's a "copy" of the bios ending at
4g. When SeaBIOS reads from this area, since vmd doesn't perform
mmio yet, guests terminate with an unhandled fault.

Carve out some space ending at 4g and copy the bios there. Technically
we could load garbage there, but give SeaBIOS what it wants for
now.

ok mlarkin@