This includes adding it to the nodedev parser and formatter, docs, and
test.
An example of the new iommuGroup element that is a part of the output
from "virsh nodedev-dumpxml" (virNodeDeviceGetXMLDesc()):
<device>
<name>pci_0000_02_00_1</name>
<capability type='pci'>
...
<iommuGroup number='12'>
<address domain='0x0000' bus='0x02' slot='0x00' function='0x0'/>
<address domain='0x0000' bus='0x02' slot='0x00' function='0x1'/>
</iommuGroup>
</capability>
</device>
