oftest.py: flush stdout before running test
OVSDriver: handle errors when parsing packet-out data
OVSDriver: ignore set-field actions on nonexistent headers
OVSDriver: improve error messages when upcall init fails
OVSDriver: improve indigo_port_stats_get error handling

The netlink callbacks object is now preallocated. Also uses the netlink message
freelist which will not fail.
Merge into master from pull request #2:
Improve error handling (https://github.com/floodlight/ivs/pull/2)