cpu: Use QTAILQ for CPU list

Introduce CPU_FOREACH(), CPU_FOREACH_SAFE() and CPU_NEXT() shorthand
macros.

Signed-off-by: Andreas Färber <afaerber@suse.de>
diff --git a/hw/i386/kvmvapic.c b/hw/i386/kvmvapic.c
index 15beb80..d3a6fbe 100644
--- a/hw/i386/kvmvapic.c
+++ b/hw/i386/kvmvapic.c
@@ -498,7 +498,7 @@
     X86CPU *cpu;
     CPUX86State *env;
 
-    for (cs = first_cpu; cs != NULL; cs = cs->next_cpu) {
+    CPU_FOREACH(cs) {
         cpu = X86_CPU(cs);
         env = &cpu->env;
         info.apic = env->apic_state;