Change ConductorManager to self.db when record cold_migrate event

In conductor manager, when cold migrate an instance, conductor
manager will record event of cold_migrate.

When record this event, conductor manager pass ConductorManager but
not self.db as parameter for compute_utils.EventReporter(), as we
can access the db directly from conductor manager so going through
ConductorManager as a proxy is unnecessary.

Change-Id: Ic52228c2550294c2df37d10d36637e4d0e3a1a30
Merge "Change ConductorManager to self.db when record cold_migrate event"