hw/i386/isapc: reject modern x86 CPU models

It was originally intended as part of the work to split out the isapc machine
from the pc machine to restrict the isapc machine to using only 32-bit CPUs.
Before the 10.2 release it was decided in commit 329e36af94 ("hw/i386/isapc.c:
warn rather than reject modern x86 CPU models") to follow the official
deprecation process due to concerns this would have an adverse effect on users.

Now that the deprecation period has expired and there have been no reported
problems, update the isapc machine to reject modern x86 CPUs by reverting
commit 329e36af94 ("hw/i386/isapc.c: warn rather than reject modern x86 CPU
models").

Signed-off-by: Mark Cave-Ayland <mark.caveayland@nutanix.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@oss.qualcomm.com>
Message-ID: <20260824111054.575149-1-mark.caveayland@nutanix.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@oss.qualcomm.com>