temporary fix for on_vcpu

Recent changes made on_vcpu hit the abort() path, even with the IO thread
disabled. This is because cpu_single_env is no longer set when we call this
function. Although the correct fix is a little bit more complicated that that,
the recent thread in which I proposed qemu_queue_work (which fixes that, btw),
is likely to go on a quite different direction.

So for the benefit of those using guest debugging, I'm proposing this simple
fix in the interim.

Signed-off-by: Glauber Costa <glommer@redhat.com>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>

kvm-all.c

@@ -905,11 +905,15 @@ void kvm_setup_guest_memory(void *start, size_t size)
 #ifdef KVM_CAP_SET_GUEST_DEBUG
 static void on_vcpu(CPUState *env, void (*func)(void *data), void *data)
 {
+#ifdef CONFIG_IOTHREAD
     if (env == cpu_single_env) {
         func(data);
         return;
     }
     abort();
+#else
+    func(data);
+#endif
 }
 
 struct kvm_sw_breakpoint *kvm_find_sw_breakpoint(CPUState *env,