add assertions on the owner of a QemuMutex

These are already present in the Win32 implementation, add them to
the pthread wrappers as well.  Use PTHREAD_MUTEX_ERRORCHECK for mutex
operations. Later we'll add tracking of the owner for cond_signal/broadcast.
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>
Signed-off-by: NBlue Swirl <blauwirbel@gmail.com>

qemu-thread-posix.c

@@ -28,8 +28,12 @@ static void error_exit(int err, const char *msg)
 void qemu_mutex_init(QemuMutex *mutex)
 {
     int err;
+    pthread_mutexattr_t mutexattr;
 
-    err = pthread_mutex_init(&mutex->lock, NULL);
+    pthread_mutexattr_init(&mutexattr);
+    pthread_mutexattr_settype(&mutexattr, PTHREAD_MUTEX_ERRORCHECK);
+    err = pthread_mutex_init(&mutex->lock, &mutexattr);
+    pthread_mutexattr_destroy(&mutexattr);
     if (err)
         error_exit(err, __func__);
 }