Since commit 83c9578fcc was merged, the
SR-IOV mechanism driver requires an agent in the compute nodes. This
change makes the SR-IOV mechanism driver to inherit from
SimpleAgentMechanismDriverBase, like all the other agent based ML2
drivers
Change-Id: Icffc097a42dd71546b97bb7b43edd1e205c9dd15
Closes-Bug: #1578274