Neutron fixups from ML2 testing

1. Shutdown virbr0 if running. This interface is created by libvirt and currently we only disable it from starting at boot, but do not shutdown the running interface.
2. Move root_helper into the [AGENT] section of neutron/l3_agent.ini to quelch a warning.
3. Patch Neutron Open vSwitch kernel module version detection so it works on new mainline kernels.