Fix a double free error in vr_message_process_response

This patch fixes a double free error and introduces
user land unit test for the vrouter code. The cmocka
unit test framework is used for that purpose.

A first unit test is provided that check this fixes.

Change-Id: Ic8c84d076d078a87adaf6d4398373c368db0361c
Closes-bug: #1394147
Merge "Fix a double free error in vr_message_process_response"