Comment 2 for bug 1393415

Reviewed: https://review.openstack.org/134966
Committed: https://git.openstack.org/cgit/openstack/ceilometer/commit/?id=98b43aad172c894d3c4e288e40eaa3c4f822d47f
Submitter: Jenkins
Branch: master

commit 98b43aad172c894d3c4e288e40eaa3c4f822d47f
Author: ZhiQiang Fan <email address hidden>
Date: Mon Nov 17 23:14:03 2014 +0800

    Return a meaningful value or raise an excpetion for libvirt

    Currently, there are some cases libvirt cannot inspect instance's memory
    usage, and it is a normal behavior so libvirt just logs messages and
    returns None. However, memory pollster doesn't check return value, so
    AttributeError will be raised in such case, which leading to unnecessary
    exception log messages in every cycle and for each instance, this will
    bother cloud operator.

    This patch uses virt.inspector exceptions for libvirt, so when there is no
    valid value can be returned, we raise an exception, pollsters can catch
    those exceptions and log proper messages. Since all of them are non-fatal
    exception, log level is set to warn, which is excatly same as previous.

    Note, this patch refactors some code of libvirt to suit this change.

    Change-Id: I2c94aab90e827c75a602403d3c64fd6f67f73007
    Closes-Bug: #1393415