vdpa: conditionally read STATUS in config space

The spec says:
    status only exists if VIRTIO_NET_F_STATUS is set

Similar to MAC and MTU, vdpa_dev_net_config_fill() should read
STATUS conditionally depending on the feature bits.

Signed-off-by: Si-Wei Liu <si-wei.liu@oracle.com>
Reviewed-by: Parav Pandit <parav@nvidia.com>
Reviewed-by: Eli Cohen <elic@nvidia.com>
Message-Id: <1675725124-7375-3-git-send-email-si-wei.liu@oracle.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>