Fix policing performance issues with VIFs.

Fix policing performance issues with VIFs.

Policing is configured with the "tc" command.  By default, it picks up
the MTU from the interface having policy applied.  When a guest operating
systems is configured for segmentation offloading, the packets handed to
DOM0 may be substantially larger than the MTU.  The policing code was
dropping these packets, which caused performance to dive.  We now
configure policing with an MTU of 64K, which solves the problem.

Thanks to Ben for diagnosing the problem.