net/vhost-vdpa.c will need functions that are declared in
vhost-shadow-virtqueue.c, that needs functions of virtio-net.c.

Copy the vhost-vdpa-stub.c code so
only the constructor net_init_vhost_vdpa needs to be defined.

Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>