Catch InstanceNotFound exceptions for V2 API virtual interfaces

Use the common.get_instance method to get the instance object
in the V2 API virtual interfaces extension rather than call
the compute api directly. The utility function
handes InstanceNotFound exceptions and tranlsates them to HTTPNotFound
rather than letting them propagate up causing a traceback.

Co-Authored-By: Michael Still <mikal@stillhq.com>

Change-Id: I8abee22c53195926e3d2af1da9b14cd9d3608e63
Closes-Bug: 1270804
Merge "Catch InstanceNotFound exceptions for V2 API virtual interfaces"