hw/ppc: free env->tb_env in spapr_unrealize_vcpu()

hw/ppc: free env->tb_env in spapr_unrealize_vcpu()

The timebase is allocated during spapr_realize_vcpu() and it's not
freed. This results in memory leaks when doing vcpu unplugs:

==636935==
==636935== 144 (96 direct, 48 indirect) bytes in 1 blocks are definitely lost in loss record 6
,461 of 8,135
==636935==    at 0x4897468: calloc (vg_replace_malloc.c:760)
==636935==    by 0x5077213: g_malloc0 (in /usr/lib64/libglib-2.0.so.0.6400.4)
==636935==    by 0x507757F: g_malloc0_n (in /usr/lib64/libglib-2.0.so.0.6400.4)
==636935==    by 0x93C3FB: cpu_ppc_tb_init (ppc.c:1066)
==636935==    by 0x97BC2B: spapr_realize_vcpu (spapr_cpu_core.c:268)
==636935==    by 0x97C01F: spapr_cpu_core_realize (spapr_cpu_core.c:337)
==636935==    by 0xD4626F: device_set_realized (qdev.c:531)
==636935==    by 0xD55273: property_set_bool (object.c:2273)
==636935==    by 0xD523DF: object_property_set (object.c:1408)
==636935==    by 0xD588B7: object_property_set_qobject (qom-qobject.c:28)
==636935==    by 0xD52897: object_property_set_bool (object.c:1477)
==636935==    by 0xD4579B: qdev_realize (qdev.c:333)
==636935==

This patch adds a cpu_ppc_tb_free() helper in hw/ppc/ppc.c to allow us
to free the timebase. This leak is then solved by calling
cpu_ppc_tb_free() in spapr_unrealize_vcpu().

Fixes: 6f4b5c3ec590 ("spapr: CPU hot unplug support")
Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Reviewed-by: Cédric Le Goater <clg@kaod.org>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Message-Id: <20220329124545.529145-2-danielhb413@gmail.com>
Signed-off-by: Cédric Le Goater <clg@kaod.org>