Description of problem:

libvirt doesn't support setting iommu for virtio-mem device

As libvirt doc said:
https://libvirt.org/formatdomain.html#virtio-related-options
```
QEMU's virtio devices have some attributes related to the virtio transport under the driver element: The iommu attribute enables the use of emulated IOMMU by the device. The attribute ats controls the Address Translation Service support for PCIe devices. This is needed to make use of IOTLB support (see IOMMU devices). Possible values are on or off. Since 3.5.0
```
E.g. Set <driver iommu='on'>
```
  <devices>
    <memballoon model='virtio'>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x02' function='0x0'/>
      <stats period='10'/>
      <driver iommu='on' ats='on'/>
    </memballoon>
  </devices>
```

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