There are certain cases where cpu context requests a vm stop, such as
-ENOSPC handling.

IMO its simpler to handle vmstop only through the iothread.

Note there is change in behaviour: now the cpu thread which requested vm_stop
will actually only stop when it exits back to cpu_main_loop. It might cause
further damage in between vmstop and cpu_main_loop.

Index: trunk/vl.c
===================================================================
--- trunk.orig/vl.c
+++ trunk/vl.c
@@ -3550,7 +3550,22 @@ void vm_start(void)
     }
 }
 
-void vm_stop(int reason)
+static int vmstop_requested;
+
+static int qemu_vmstop_requested(void)
+{
+    int r = vmstop_requested;
+    vmstop_requested = 0;
+    return r;
+}
+
+static void qemu_system_vmstop_request(int reason)
+{
+    vmstop_requested = reason;
+    main_loop_break();
+}
+
+static void __vm_stop(int reason)
 {
     if (vm_running) {
         cpu_disable_ticks();
@@ -3560,6 +3575,21 @@ void vm_stop(int reason)
     }
 }
 
+void vm_stop(int reason)
+{
+    QemuThread me;
+    qemu_thread_self(&me);
+
+    if (!qemu_thread_equal(&me, &io_thread)) {
+        qemu_system_vmstop_request(reason);
+        /* make sure we can't return to cpu_exec */
+        if (cpu_single_env)
+            cpu_single_env->stop = 1;
+        return;
+    }
+    __vm_stop(reason);
+}
+
 /* reset/shutdown handler */
 
 typedef struct QEMUResetEntry {
@@ -3757,6 +3787,8 @@ static int cpu_can_run(CPUState *env)
         return 0;
     if (reset_requested)
         return 0;
+    if (vmstop_requested)
+        return 0;
     return 1;
 }
 
@@ -4203,6 +4235,8 @@ static void qemu_init_state(void)
 
 static void main_loop(void)
 {
+    int r;
+
     qemu_thread_self(&io_thread);
     setup_iothread_fd();
 
@@ -4219,12 +4253,14 @@ static void main_loop(void)
                 no_shutdown = 0;
             else
                     break;
-        } else if (qemu_powerdown_requested())
+        } else if (qemu_powerdown_requested()) {
             qemu_system_powerdown();
-        else if (qemu_reset_requested()) {
+        } else if (qemu_reset_requested()) {
             pause_all_vcpus();
             qemu_system_reset();
             resume_all_vcpus();
+        } else if ((r = qemu_vmstop_requested())) {
+            vm_stop(r);
         }
     }
 }