nodedev: add iommuGroup to node device object

nodedev: add iommuGroup to node device object

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>