dpdk: allow specifying of `rss_hf` flags in config
Since we can only test a small subset of cards it would be useful to expose this field to the config, even if it is done as an advanced "raw" field: that would just take a hex (
0xFFFF) notation. We can make the DumpRssFlags function a bit nicer to assist the user.
The alternative is that each user would need to edit the code for a NIC that we haven't tested, so it seems nice to avoid this.