iommu/arm-smmu-v3-kvm: Enable nesting

As now the hypervisor controls the command queue, stream table,
and shadows the stage-2 page table, we enable stage-2 translation
in case the host puts an STE in bypass or stage-1.

The hypervisor will control the S2 part of the STE and sanitize it.

Signed-off-by: Mostafa Saleh <smostafa@google.com>