exec: Don't use cpu_index to detect if cpu_exec_init()'s been called

Instead use QTAIL's tqe_prev field to detect if cpu's been
placed in list by cpu_exec_init() which is always set if
QTAIL element is in list.

Fixes SIGSEGV on failure path in case cpu_index is assigned
by board and cpu.relalize() fails before cpu_exec_init() is called.

In follow up patches, cpu_index will be assigned by boards that
support cpu hot(un)plug and need stable cpu_index that doesn't
depend on order cpus are created/removed.

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Reported-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>

exec.c

@@ -643,13 +643,14 @@ void cpu_exec_exit(CPUState *cpu)
     CPUClass *cc = CPU_GET_CLASS(cpu);
 
     cpu_list_lock();
-    if (cpu->cpu_index == -1) {
-        /* cpu_index was never allocated by this @cpu or was already freed. */
+    if (cpu->node.tqe_prev == NULL) {
+        /* there is nothing to undo since cpu_exec_init() hasn't been called */
         cpu_list_unlock();
         return;
     }
 
     QTAILQ_REMOVE(&cpus, cpu, node);
+    cpu->node.tqe_prev = NULL;
     cpu_release_index(cpu);
     cpu->cpu_index = -1;
     cpu_list_unlock();