Fix AttributeError(s) from get_v4/6_ips_by_interface

Add some defensive code to prevent AttributeError especially
when virtual_interface_get_by_uuid returns None. Add some basic
tests for good and bad data returned from
virtual_interface_get_by_uuid

Change-Id: I0ee1bb2087a6ed2619ac343534e4ade84e08d9bd
Closes-Bug: 1246087
Merge "Fix AttributeError(s) from get_v4/6_ips_by_interface"