docs: vhost-user: clean up request/reply description

docs: vhost-user: clean up request/reply description

It is not necessary to mention which side is sending/receiving
each payload; it is more interesting to say which is the request
and which is the reply.  This also matches what vhost-user-gpu.rst
already does.

While at it, ensure that all messages list both the request and
the reply payload.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20210226143413.188046-2-pbonzini@redhat.com>
Message-Id: <20220321153037.3622127-5-alex.bennee@linaro.org>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>