monitor: Use qemu_get_cpu() in monitor_set_cpu()

No functional change, just a reduction of CPU loops.

The mon_cpu field is left untouched for now since changing that requires
a number of larger prerequisites, including cpu_synchronize_state() and
mon_get_cpu().

Reviewed-by: Luiz Capitulino <lcapitulino@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>

monitor.c

@@ -855,17 +855,14 @@ EventInfoList *qmp_query_events(Error **errp)
 /* set the current CPU defined by the user */
 int monitor_set_cpu(int cpu_index)
 {
-    CPUArchState *env;
     CPUState *cpu;
 
-    for (env = first_cpu; env != NULL; env = env->next_cpu) {
-        cpu = ENV_GET_CPU(env);
-        if (cpu->cpu_index == cpu_index) {
-            cur_mon->mon_cpu = env;
-            return 0;
-        }
+    cpu = qemu_get_cpu(cpu_index);
+    if (cpu == NULL) {
+        return -1;
     }
-    return -1;
+    cur_mon->mon_cpu = cpu->env_ptr;
+    return 0;
 }
 
 static CPUArchState *mon_get_cpu(void)