Add missing tests for db.virtual_interface_* methods

There was no tests in test_db_api for virtual_interface_* methods.

Add tests to ensure that all works.

This allow us to refactor these methods to use soft_delete instead of
delete.

Found bug 1156227 in sqlite we have lost UC on virtual_interfaces.address
after 152_* migration. To fix it we should add new db migration, so it will be
fixed in Havana-1.

blueprint db-api-tests

Change-Id: Ide03163b230762d65bf3cd7b5fe8f0a303fea5ae
Merge "Add missing tests for db.virtual_interface_* methods"