vdpa_sim: switch to use __vdpa_alloc_device()

This allows us to control the allocation size of the structure.

Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Acked-by: Eugenio Pérez <eperezma@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>
Message-Id: <20221223055548.27810-2-jasowang@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>