net/virtio_user: fix first queue pair without multiqueue

net/virtio_user: fix first queue pair without multiqueue

When virtio_user is used with OVS-DPDK (with mq disabled), it cannot
receive any packets. This is because no queue is enabled at all when
mq is disabled.

To fix it, we should consistently make sure the 1st queue is enabled,
which is also the behaviour QEMU takes.

Fixes: 37a7eb2ae816 ("net/virtio-user: add device emulation layer")

Reported-by: Ning Li <lining18@jd.com>
Signed-off-by: Jianfeng Tan <jianfeng.tan@intel.com>
Acked-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>