hw/core/machine: Add the missing delimiter in cpu_slot_to_string()

hw/core/machine: Add the missing delimiter in cpu_slot_to_string()

The expected output string from cpu_slot_to_string() ought to be
like "socket-id: *, die-id: *, core-id: *, thread-id: *", so add
the missing ", " before "die-id". This affects the readability
of the error message.

Fixes: 176d2cda0d ("i386/cpu: Consolidate die-id validity in smp context")
Signed-off-by: Yanan Wang <wangyanan55@huawei.com>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Message-Id: <20211008075040.18028-1-wangyanan55@huawei.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>