Fix init of pci_stats in resource tracker

Because pci_stats is list of dict, i.e., [{}, {}, ...],
if pci_stats is null, its field should be '[]' not '{}'.

When making object models, we found this bug in PCI stuffs.

Related to blueprint pci-extra-info.

Change-Id: I06564cb541b050060e87fd3d334299df66f23dec
Merge "Fix init of pci_stats in resource tracker"