Fix PortNotFound error during update_device_up for DVR

An agent's request to update the ARP entry for a VM port
may come after a deletion request has been processed,
resulting in a PortNotFound exception being raised.

This patch takes care of this condition. A test has
been added, which required a minor refactoring of the
test case class, in order to accommodate the use of
side effects for the objects being mocked.

Closes-bug: #1356120

Change-Id: I40d635bcf47c683663cb4dedf20323902dff2c7f
Merge "Fix PortNotFound error during update_device_up for DVR"