LinuxBIOS writes the IRQ routing table (PIRQ) to 0xf000 and then reads
it back to verify the write. Currently qemu maps the top 128 KB of the
BIOS into ISA address space (0xe000 - 0xffff) as ROM, which causes the
write to fail, preventing Linux from finding interrupt routing info.

This patch changes qemu to map the BIOS into ISA address space as RAM
instead of ROM, allowing LinuxBIOS to run on qemu with no further
modifications (although the DRAM size is still not detected properly).

I've verified that Windows 2000 and Linux still boot with the Bochs
BIOS with this patch applied.

--Ed