IVSSwitch: support batch shutdown

Not a single command like the OVS one, but it still greatly decreases the
shutdown time.

This does assume that stop() is called after batchShutdown(), which is true in
the current mininet code.
net: allow batch shutdown of multiple types of switches

Each switch class will be called to shutdown its own instances.
Merge pull request #309 from rlane/ivs-batch-shutdown

support batch shutdown for IVS