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 too.  Document more clearly what
is going on.

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