In terms of other flash devices, I don't think it's that simple. Flash is
tied to the mobo layout so I don't think index> 0 really makes sense unless
you allow a specific mapping address. I doubt that's terribly useful.
I think VM's have a different situation than real hardware. I'm not
sure an all ROM or all flash decision will work well for qemu. In
most cases it may work better to make a ROM image available just below
4GB, and add a flash image below this ROM.
This allows the qemu's firmware to be updated as usual in
${prefix}/share/bios.bin, but still allows a flash memory to be
available below this. (The flash below the ROM could be used only for
storing UEFI variables.)
Otherwise when a new qemu is released along with a new firmware image,
the VM instance using writable flash will continue to use the old
firmware image.