net/virtio-user: add multiple queues in device emulation

net/virtio-user: add multiple queues in device emulation

The main purpose of this patch is to enable multi-queue. But
multi-queue requires ctrl-queue so that driver can send how many
queues will be enabled through ctrl-queue messages.

So we partially implement ctrl-queue to handle control command
with class of VIRTIO_NET_CTRL_MQ and with cmd of
VIRTIO_NET_CTRL_MQ_VQ_PAIRS_SET to handle mq support. This patch
provides a function, virtio_user_handle_cq(), for driver to handle
ctrl-queue messages.

Besides, multi-queue requires VIRTIO_NET_F_MQ and VIRTIO_NET_F_CTRL_VQ
are enabled when we do feature negotiation.

Signed-off-by: Jianfeng Tan <jianfeng.tan@intel.com>
Acked-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>