Avoid bandwidth usage db query in notifications when the virt driver does not support collecting such data

[Balazs Gibizer]

The notify_usage_exist() function always query the DB to get the bandwidth usage data. But such data is only available if the CONF.bandwidth_poll_interval is set to a positive number (600 is the default) and the virt driver supports such data collection. Today only xenapi driver supports this data collection. So it would make sense to avoid the DB query when it is known that such data is not available.

[Balazs Gibizer (balazs-gibizer)]

There is an ML post asking about the support of the whole bandwidth usage codepath in the xenapi driver. http://lists.openstack.org/pipermail/openstack-dev/2018-April/129397.html