migration/savevm: release gslist after dump_vmstate_json

'list' forgot to free at the end of dump_vmstate_json_to_file(), although it's called only once, but seems like a clean code.

Fix the leak as follow:
Direct leak of 16 byte(s) in 1 object(s) allocated from:
    #0 0x7fb946abd768 in __interceptor_malloc (/lib64/libasan.so.5+0xef768)
    #1 0x7fb945eca445 in g_malloc (/lib64/libglib-2.0.so.0+0x52445)
    #2 0x7fb945ee2066 in g_slice_alloc (/lib64/libglib-2.0.so.0+0x6a066)
    #3 0x7fb945ee3139 in g_slist_prepend (/lib64/libglib-2.0.so.0+0x6b139)
    #4 0x5585db591581 in object_class_get_list_tramp /mnt/sdb/qemu-new/qemu/qom/object.c:1084
    #5 0x5585db590f66 in object_class_foreach_tramp /mnt/sdb/qemu-new/qemu/qom/object.c:1028
    #6 0x7fb945eb35f7 in g_hash_table_foreach (/lib64/libglib-2.0.so.0+0x3b5f7)
    #7 0x5585db59110c in object_class_foreach /mnt/sdb/qemu-new/qemu/qom/object.c:1038
    #8 0x5585db5916b6 in object_class_get_list /mnt/sdb/qemu-new/qemu/qom/object.c:1092
    #9 0x5585db335ca0 in dump_vmstate_json_to_file /mnt/sdb/qemu-new/qemu/migration/savevm.c:638
    #10 0x5585daa5bcbf in main /mnt/sdb/qemu-new/qemu/vl.c:4420
    #11 0x7fb941204812 in __libc_start_main ../csu/libc-start.c:308
    #12 0x5585da29420d in _start (/mnt/sdb/qemu-new/qemu/build/x86_64-softmmu/qemu-system-x86_64+0x27f020d)

Indirect leak of 7472 byte(s) in 467 object(s) allocated from:
    #0 0x7fb946abd768 in __interceptor_malloc (/lib64/libasan.so.5+0xef768)
    #1 0x7fb945eca445 in g_malloc (/lib64/libglib-2.0.so.0+0x52445)
    #2 0x7fb945ee2066 in g_slice_alloc (/lib64/libglib-2.0.so.0+0x6a066)
    #3 0x7fb945ee3139 in g_slist_prepend (/lib64/libglib-2.0.so.0+0x6b139)
    #4 0x5585db591581 in object_class_get_list_tramp /mnt/sdb/qemu-new/qemu/qom/object.c:1084
    #5 0x5585db590f66 in object_class_foreach_tramp /mnt/sdb/qemu-new/qemu/qom/object.c:1028
    #6 0x7fb945eb35f7 in g_hash_table_foreach (/lib64/libglib-2.0.so.0+0x3b5f7)
    #7 0x5585db59110c in object_class_foreach /mnt/sdb/qemu-new/qemu/qom/object.c:1038
    #8 0x5585db5916b6 in object_class_get_list /mnt/sdb/qemu-new/qemu/qom/object.c:1092
    #9 0x5585db335ca0 in dump_vmstate_json_to_file /mnt/sdb/qemu-new/qemu/migration/savevm.c:638
    #10 0x5585daa5bcbf in main /mnt/sdb/qemu-new/qemu/vl.c:4420
    #11 0x7fb941204812 in __libc_start_main ../csu/libc-start.c:308
    #12 0x5585da29420d in _start (/mnt/sdb/qemu-new/qemu/build/x86_64-softmmu/qemu-system-x86_64+0x27f020d)

Reported-by: Euler Robot <euler.robot@huawei.com>
Signed-off-by: Pan Nengyuan <pannengyuan@huawei.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>

migration/savevm.c

@@ -665,6 +665,7 @@ void dump_vmstate_json_to_file(FILE *out_file)
     }
     fprintf(out_file, "\n}\n");
     fclose(out_file);
+    g_slist_free(list);
 }
 
 static uint32_t calculate_new_instance_id(const char *idstr)