vhost: implement vhost_force_iommu method

use the vhost_force_iommu callback to force enable feature bit VIRTIO_F_IOMMU_PLATFORM

Signed-off-by: Cindy Lu <lulu@redhat.com>
Message-Id: <20200701145538.22333-12-lulu@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>