OVSDriver: workaround valgrind not understanding ethtool

Valgrind complains that the fields populated by the ethtool ioctl are
uninitialized, when actually the kernel wrote to them. Work around this by
initializing the result before the ioctl.
OVSDriver: clean up link cache at exit

Valgrind was marking this as "possibly lost", which is untrue but worth working
around.
OVSDriver: clean up upcall threads at exit

This frees the majority of the reachable memory, which makes reading valgrind
output easier.

The upcall threads now wake up once per second to see if they should exit.
Merge into master from pull request #25:
valgrind fixes (https://github.com/floodlight/ivs/pull/25)