hw/i386/x86: Extract x86_isa_bios_init() from x86_bios_rom_init()

hw/i386/x86: Extract x86_isa_bios_init() from x86_bios_rom_init()

The function is inspired by pc_isa_bios_init() and should eventually replace it.
Using x86_isa_bios_init() rather than pc_isa_bios_init() fixes pflash commands
to work in the isa-bios region.

While at it convert the magic number 0x100000 (== 1MiB) to increase readability.

Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Bernhard Beschow <shentey@gmail.com>
Message-ID: <20240508175507.22270-6-shentey@gmail.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>