virtio_pci_modern: pass cmd as an identification token

In preparation to asynchronous admin queue processing, pass cmd pointer
as a data arg to virtqueue_add_sgs().

Signed-off-by: Jiri Pirko <jiri@nvidia.com>
Message-Id: <20240716113552.80599-12-jiri@resnulli.us>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>