qemu-thread: use acquire/release to clarify semantics of QemuEvent

Do not use the somewhat mysterious atomic_mb_read/atomic_mb_set,
instead make sure that the operations on QemuEvent are annotated
with the desired acquire and release semantics.

In particular, qemu_event_set wakes up the waiting thread, so it must
be a release from the POV of the waker (compare with qemu_mutex_unlock).
And it actually needs a full barrier, because that's the only thing that
provides something like a "load-release".

Use smp_mb_acquire until we have atomic_load_acquire and
atomic_store_release in atomic.h.

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