agent: Disable destruction of flow device on FreeBSD

The flow device created by vRouter module is removed and recreated
by userspace Agent with the ID obtained from vRouter module; this
does not work correctly on FreeBSD due to difference in device
node creation. It is enouth to disable the destruction and use
the device created by vrouter module instead.
Merge pull request #10 from Semihalf/fix-flow-device-code

agent: Disable destruction of flow device on FreeBSD