vDPA/ifcvf: detect and use the onboard number of queues directly

To enable this multi-queue feature for ifcvf, this commit
intends to detect and use the onboard number of queues
directly than IFCVF_MAX_QUEUE_PAIRS = 1 (removed)

Signed-off-by: Zhu Lingshan <lingshan.zhu@intel.com>
Link: https://lore.kernel.org/r/20210818095714.3220-2-lingshan.zhu@intel.com
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>