Description of problem:

Libvirt and qemu have supported vIOMMU emulation for some time now.
nova curretly does not expose the vIOMMU as a configurable paramter.
using the q35 machine type and vIOMMU feature can enable better securty in
the guest when using dpdk or pci device passsthough by allowing the viommu
to work with the host iommu to restrict the the meory the passthough or dpdk
device can acess in the guest.

in some case this can also improve performance by reducing the cost of phyical page traslation.

by enableing support for vIOMMU emultaiton in the guest this will also enable
futrue work to allow nested sriov passhtough or other more advanced configurations that require a IOMMU to funciton correcly such as using vfio-pci in the guest.

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