virtio: support multiple queues

virtio: support multiple queues

This patch supports multiple queues feature in DPDK based virtio-net frontend.
It firstly gets max queue number of virtio-net from virtio PCI configuration and
then send command to negotiate the queue number with backend; When receiving and
transmitting packets, it negotiates multiple virtio-net queues which serve RX/TX;
To utilize this feature, the backend also need support multiple queues feature
and enable it.

Signed-off-by: Ouyang Changchun <changchun.ouyang@intel.com>
Tested-by: Waterman Cao <waterman.cao@intel.com>
Acked-by: Thomas Monjalon <thomas.monjalon@6wind.com>