vhost-vdpa: allow passing opened vhostfd to vhost-vdpa

vhost-vdpa: allow passing opened vhostfd to vhost-vdpa

Similar to other vhost backends, vhostfd can be passed to vhost-vdpa
backend as another parameter to instantiate vhost-vdpa net client.
This would benefit the use case where only open file descriptors, as
opposed to raw vhost-vdpa device paths, are accessible from the QEMU
process.

(qemu) netdev_add type=vhost-vdpa,vhostfd=61,id=vhost-vdpa1

Signed-off-by: Si-Wei Liu <si-wei.liu@oracle.com>
Acked-by: Eugenio Pérez <eperezma@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>