Resource tracking updates PCI stats when they are not supported by the underlying driver

[Gary Kotton]

At the moment only libvirt supports resource tracking. In the case that the host does not report the pci_stats then the data structure will be updated and there will be writes to the database. This will be done even though the data is not retuned from the host.
It is a waste of cycle and valuable resoures

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/47744

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/47744
Committed: http://github.com/openstack/nova/commit/6bb5c7302f67d130af2c92da0e1b98164ca46697
Submitter: Jenkins
Branch: master

commit 6bb5c7302f67d130af2c92da0e1b98164ca46697
Author: Gary Kotton <email address hidden>
Date: Sun Sep 22 07:51:44 2013 -0700

    Only update PCI stats if they are reported from the host

    Not all drivers and hosts may report PCI stats. These should only
    be updated if they are actually reported. This will save valuable cycles
    and not have any unnecessary database access.

    Fixes bug: 1228872

    Change-Id: I3e0170638f3820422dbdea5dfb9e02b2de2ceacd