Fix typo in _update_usage_from_migrations
Fix typo in _update_usage_from_migrations

The error message in _update_usage_from_migrations() was using
the uuid variable inappropriately.  We want to use instance.uuid
instead.

Change-Id: Ibf907bd64b57445aaf8e1428764367640c1cad20

alaski/nova,gooddata/openstack-nova,vmturbo/nova,sebrandon1/nova,mahak/nova,Juniper/nova,sebrandon1/nova,Juniper/nova,vmturbo/nova,vmturbo/nova,klmitch/nova,rajalokan/nova,hanlind/nova,jianghuaw/nova,rahulunair/nova,openstack/nova,mahak/nova,Juniper/nova,jianghuaw/nova,Juniper/nova,rajalokan/nova,klmitch/nova,alaski/nova,phenoxim/nova,jianghuaw/nova,gooddata/openstack-nova,sebrandon1/nova,mikalstill/nova,openstack/nova,phenoxim/nova,rajalokan/nova,jianghuaw/nova,rajalokan/nova,gooddata/openstack-nova,vmturbo/nova,cloudbase/nova,cloudbase/nova,hanlind/nova,mikalstill/nova,klmitch/nova,hanlind/nova,rahulunair/nova,klmitch/nova,rahulunair/nova,gooddata/openstack-nova,cloudbase/nova,mahak/nova,openstack/nova,mikalstill/nova