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.
IVSSwitch: add an option to control the --verbose flag
Merge pull request #309 from rlane/ivs-batch-shutdown

support batch shutdown for IVS
Merge pull request #310 from rlane/ivs-verbose

IVSSwitch: add an option to control the --verbose flag
slight refactoring of util/vm/build.py to make it more extensible
build.py: making ovf generation more generic
Added support for mount namespaces in bind.py. Also moved it to the node class as a host type.
fixed netParse bug that caused mininet crash when no ip prefix was specified
Merge branch 'patches/fix_bindpy' of https://github.com/cdburkard/mininet into patches/fix_bindpy
Merge pull request #313 from cdburkard/master

ovs port numbering bug fix
added code to kill stale mininet processes
added persistence option to HostWithPrivateDirs. also attached mount namespaces when mnexec -a is specified
Merge pull request #316 from cdburkard:patches/fix_remote_ip

fixed netParse bug that caused mininet crash when no ip prefix was specified
imported check_output
mountprivatedirs is no longer needed
added documentation for HostWithPrivateDirs
Whitespace changes in OVSSwitch.
Merge pull request #317 from cdburkard/patches/cleanup

added code to kill stale mininet processes
use a single mininet instance in bindpy