libvirt: fix missed test in migration
libvirt: fix missed test in migration

The method get_updated_guest_xml has a specific test to ensure all
dependent methods are well called with the well parameters.  This
commit adds an assertion to ensure update_perf_events_xml is well
called when we want to retrieve the domain xml of a guest for
migration.

Change-Id: Ifda45b824fbc819173498be606622e8e663b2626

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