vhost-vdpa: account iommu allocations

iommu allocations should be accounted in order to allow admins to
monitor and limit the amount of iommu memory.

Signed-off-by: Pasha Tatashin <pasha.tatashin@soleen.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Message-Id: <20231226182827.294158-1-pasha.tatashin@soleen.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: David Rientjes <rientjes@google.com>