cpus: be more paranoid in avoiding deadlocks

For good measure, ensure that the following sequence:

   thread 1 calls qemu_mutex_lock_iothread
   thread 2 calls qemu_mutex_lock_iothread
   VCPU thread are created
   VCPU thread enters execution loop

results in the VCPU threads letting the other two threads run
and obeying iothread_requesting_mutex even if the VCPUs are
not halted.  To do this, check iothread_requesting_mutex
before execution starts.
Tested-by: NLeon Alrae <leon.alrae@imgtec.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

cpus.c

@@ -1025,6 +1025,9 @@ static void *qemu_tcg_cpu_thread_fn(void *arg)
         }
     }
 
+    /* process any pending work */
+    exit_request = 1;
+
     while (1) {
         tcg_exec_all();
 
@@ -1115,10 +1118,11 @@ bool qemu_in_vcpu_thread(void)
 
 void qemu_mutex_lock_iothread(void)
 {
+    atomic_inc(&iothread_requesting_mutex);
     if (!tcg_enabled() || !first_cpu) {
         qemu_mutex_lock(&qemu_global_mutex);
+        atomic_dec(&iothread_requesting_mutex);
     } else {
-        atomic_inc(&iothread_requesting_mutex);
         if (qemu_mutex_trylock(&qemu_global_mutex)) {
             qemu_cpu_kick_thread(first_cpu);
             qemu_mutex_lock(&qemu_global_mutex);