qapi/machine.json: Fix incorrect description for die-id

qapi/machine.json: Fix incorrect description for die-id

In terms of scope, die-id should mean "the die number within
socket the CPU belongs to" instead of "the die number within
node/board the CPU belongs to". Fix it to avoid confusing
the Doc reader.

Fixes: 176d2cda0d ("i386/cpu: Consolidate die-id validity in smp context")
Signed-off-by: Yanan Wang <wangyanan55@huawei.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20211122032651.16064-1-wangyanan55@huawei.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>