vdpa/ifcvf: fix the calculation of queuepair

The q_pair_id to address a queue pair in the lm bar should be
calculated by queue_id / 2 rather than queue_id / nr_vring.

Fixes: 2ddae773 ("vDPA/ifcvf: detect and use the onboard number of queues directly")
Signed-off-by: Angus Chen <angus.chen@jaguarmicro.com>
Reviewed-by: Jason Wang <jasowang@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Zhu Lingshan <lingshan.zhu@intel.com>
Message-Id: <20220923091013.191-1-angus.chen@jaguarmicro.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>