qemu-thread-win32: cleanup, fix, document QemuEvent

qemu-thread-win32: cleanup, fix, document QemuEvent

QemuEvent is currently broken on ARM due to missing memory barriers
after qatomic_*().  Apart from adding the memory barrier, a closer look
reveals some unpaired memory barriers that are not really needed and
complicated the functions unnecessarily.  Also, it is relying on
a memory barrier in ResetEvent(); the barrier _ought_ to be there
but there is really no documentation about it, so make it explicit.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>