hw/i386/pc_piix.c: restrict isapc machine to 32-bit CPUs

hw/i386/pc_piix.c: restrict isapc machine to 32-bit CPUs

The isapc machine represents a legacy ISA PC with a 486 CPU. Whilst it is
possible to specify any CPU via -cpu on the command line, it makes no
sense to allow modern 64-bit CPUs to be used.

Restrict the isapc machine to the available 32-bit CPUs, taking care to
handle the case where if a user inadvertently uses either -cpu max or
-cpu host then the "best" 32-bit CPU is used (in this case the pentium3).

Signed-off-by: Mark Cave-Ayland <mark.caveayland@nutanix.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Link: https://lore.kernel.org/r/20250828111057.468712-2-mark.caveayland@nutanix.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>