virtio-net: add per-vCPU Rx/Tx support

Each virtio-net has the power of 2 queues, and is per-vCPU Rx/Tx.
Below is the layout of queue no:
0:rx0 1:tx0 2:rx1 3:tx1 ... 2N:rxN 2N+1:txN

2N:rxN and 2N+1:txN are assigned to cpuN by default.

Signed-off-by: Zhi Yong Wu <zwu.kernel@gmail.com>