provide cpu_index to env mapping

There are some people interested in, given a cpu number,
pick its CPUState. KVM is an example, although not yet in tree.
This patch provides a way of doing that.
Signed-off-by: NGlauber Costa <glommer@redhat.com>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>

cpu-all.h

@@ -743,6 +743,7 @@ int page_check_range(target_ulong start, target_ulong len, int flags);
 
 void cpu_exec_init_all(unsigned long tb_size);
 CPUState *cpu_copy(CPUState *env);
+CPUState *qemu_get_cpu(int cpu);
 
 void cpu_dump_state(CPUState *env, FILE *f,
                     int (*cpu_fprintf)(FILE *f, const char *fmt, ...),

exec.c

@@ -541,6 +541,19 @@ static int cpu_common_load(QEMUFile *f, void *opaque, int version_id)
 }
 #endif
 
+CPUState *qemu_get_cpu(int cpu)
+{
+    CPUState *env = first_cpu;
+
+    while (env) {
+        if (env->cpu_index == cpu)
+            break;
+        env = env->next_cpu;
+    }
+
+    return env;
+}
+
 void cpu_exec_init(CPUState *env)
 {
     CPUState **penv;