net/virtio: fix less queues being enabled

net/virtio: fix less queues being enabled

From the virtio spec of view, multiple-queue is always enabled/disabled
in queue pairs. DPDK somehow allows the case when Tx and Rx queue number
are different.

Currently, virtio PMD get the queue pair number from the nb_rx_queues
field, which could be an issue when Tx queue number > Rx queue number.
Say, 2 Tx queues and 1 Rx queues. This would end up with 1 quues being
enabled. Which is wrong.

The fix is straightforward. Just pick a bigger number and enable that many
of queues.

Fixes: 823ad647950a ("virtio: support multiple queues")

Signed-off-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>