Intel10G: set the rate limit on the NIC

Adds an optional `rate_limit = xxx` config option to set a hard transmit rate limit for a single VF.  the `xxx` is a simple float value expressed in Mbit/sec, range is from 10 to 10000 (10Mbps to 10Gbps).  Current tests show it at around 70% of desired value, apparently because of overhead of the packet repeater app.