netdev-dpdk: Reset RSS hash when receiving from a vhost device.

netdev-dpdk: Reset RSS hash when receiving from a vhost device.

A vhost device, being a virtual interface, doesn't provide a valid RSS
hash.  As a workaround we set the value to 0, which suggests the
datapath to recompute the hash in software.

Reported-by: Dongjun <dongj@dtdream.com>
CC: Traynor, Kevin <kevin.traynor@intel.com>
CC: Flavio Leitner <fbl@sysclose.org>
Signed-off-by: Daniele Di Proietto <diproiettod@vmware.com>
Acked-by: Flavio Leitner <fbl@sysclose.org>
Acked-by: Pravin B Shelar <pshelar@nicira.com>