i386/cpu: Fix cpu number overflow in CPUID.01H.EBX[23:16]

i386/cpu: Fix cpu number overflow in CPUID.01H.EBX[23:16]

The legacy topology enumerated by CPUID.1.EBX[23:16] is defined in SDM
Vol2:

Bits 23-16: Maximum number of addressable IDs for logical processors in
this physical package.

When threads_per_socket > 255, it will 1) overwrite bits[31:24] which is
apic_id, 2) bits [23:16] get truncated.

Specifically, if launching the VM with -smp 256, the value written to
EBX[23:16] is 0 because of data overflow. If the guest only supports
legacy topology, without V2 Extended Topology enumerated by CPUID.0x1f
or Extended Topology enumerated by CPUID.0x0b to support over 255 CPUs,
the return of the kernel invoking cpu_smt_allowed() is false and APs
(application processors) will fail to bring up. Then only CPU 0 is online,
and others are offline.

For example, launch VM via:
qemu-system-x86_64 -M q35,accel=kvm,kernel-irqchip=split \
    -cpu qemu64,cpuid-0xb=off -smp 256 -m 32G \
    -drive file=guest.img,if=none,id=virtio-disk0,format=raw \
    -device virtio-blk-pci,drive=virtio-disk0,bootindex=1 --nographic

The guest shows:
    CPU(s):               256
    On-line CPU(s) list:  0
    Off-line CPU(s) list: 1-255

To avoid this issue caused by overflow, limit the max value written to
EBX[23:16] to 255 as the HW does.

Cc: qemu-stable@nongnu.org
Reviewed-by: Xiaoyao Li <xiaoyao.li@intel.com>
Signed-off-by: Qian Wen <qian.wen@intel.com>
Signed-off-by: Zhao Liu <zhao1.liu@intel.com>
Link: https://lore.kernel.org/r/20250714080859.1960104-6-zhao1.liu@intel.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>