vdpa: conditionally fill max max queue pair for stats

For the device without multiqueue feature, we will read 0 as
max_virtqueue_pairs from the config. So if we fill
VDPA_ATTR_DEV_NET_CFG_MAX_VQP with the value we read from the config
we will confuse the user.

Fixing this by only filling the value when multiqueue is offered by
the device so userspace can assume 1 when the attr is not provided.

Fixes: 13b00b13("vdpa: Add support for querying vendor statistics")
Cc: Eli Cohen <elic@nvidia.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>
Message-Id: <20220907060110.4511-1-jasowang@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Eli Cohen <elic@nvidia.com>