Remove the vhost tap if the physical interface corresponsing to a vhost
interface changes

If before a vrouter soft reset the physical interface was different and
post reset agent adds a different physical interface (corresponding to a
particular vhost), then a system crash is possible subsequently when
somebody tries to remove the module. (This situation can happen  because
interfaces mentioned in agent_param and contrail-vrouter-agent.conf are
different. Unlikely, but possible).

The system crash is because during reset we install a handler on the
physical interface that directly shunts the packet to vhost interface in
the kernel. Post reset, if the interface changes, we don't remove this tap.
If we then remove the vrouter module, the tap points to the module function
and hence an invalid memory.

Change-Id: I28d4a7691d7caea81f849110edf7c7f7170b0bc5
Closes-BUG: #1394473
Merge "Remove the vhost tap if the physical interface corresponsing to a vhost interface changes"