util/qht: Document memory ordering assumptions

util/qht: Document memory ordering assumptions

It is naturally expected that some memory ordering should be provided
around qht_insert() and qht_lookup(). Document these assumptions in the
header file and put some comments in the source to denote how that
memory ordering requirements are fulfilled.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
[Sergey Fedorov: commit title and message provided;
comment on qht_remove() elided]
Signed-off-by: Sergey Fedorov <serge.fdrv@gmail.com>
Message-Id: <20160715175852.30749-2-sergey.fedorov@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>