vdpa: Add missing comment for virtqueue count

Add missing comment for number of virtqueue.

Signed-off-by: Parav Pandit <parav@nvidia.com>
Reviewed-by: Eli Cohen <elic@nvidia.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Link: https://lore.kernel.org/r/20201112064005.349268-2-parav@nvidia.com


Signed-off-by: Michael S. Tsirkin <mst@redhat.com>