Clean up check for migration 213

Migration 213 for PCI device needs to check whether the pci_stats column
and the PCI device table exist or not after and before migration.

For pci_stats column check, interface assertColumnExists and
assertColumnNotExists could be used to simplify.

PS: remove an unnecessary word 'image_id' in models.py.

Change-Id: I7f1ba266523bca68c950bd76449324cece09393f
Signed-off-by: Shane Wang <shane.wang@intel.com>
Merge "Clean up check for migration 213"