qemu-thread: add QemuEvent

This emulates Win32 manual-reset events using futexes or conditional
variables.  Typical ways to use them are with multi-producer,
single-consumer data structures, to test for a complex condition whose
elements come from different threads:

    for (;;) {
        qemu_event_reset(ev);
        ... test complex condition ...
        if (condition is true) {
            break;
        }
        qemu_event_wait(ev);
    }

Or more efficiently (but with some duplication):

    ... evaluate condition ...
    while (!condition) {
        qemu_event_reset(ev);
        ... evaluate condition ...
        if (!condition) {
            qemu_event_wait(ev);
            ... evaluate condition ...
        }
    }

QemuEvent provides a very fast userspace path in the common case when
no other thread is waiting, or the event is not changing state.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

include/qemu/thread-posix.h

@@ -21,6 +21,14 @@ struct QemuSemaphore {
 #endif
 };
 
+struct QemuEvent {
+#ifndef __linux__
+    pthread_mutex_t lock;
+    pthread_cond_t cond;
+#endif
+    unsigned value;
+};
+
 struct QemuThread {
     pthread_t thread;
 };