This patch switches to let net_vhost_vdpa_init() to return
NetClientState *. This is used for the callers to allocate multiqueue
NetClientState for multiqueue support.

Signed-off-by: Jason Wang <jasowang@redhat.com>
Message-Id: <20211020045600.16082-5-jasowang@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>