Prior to commit 7d5bf484 (first appearing in libvirt 1.2.2), the
status XML of a domain's interface was missing a lot of important
information; mainly it just output the config of the interface, plus
the name of the tap device and qemu device alias. Commit 7d5bf484
changed the status XML to include many important bits of information
that were required to make network "hook" scripts useful - bandwidth
information, vlan tag, the name of the bridge (or physical device in
the case of macvtap) that the tap/macvtap device was attached to - the
commit log for 7d5bf484 has a very detailed explanation of the
change. For quick reference - in the example given there, prior to the
change, status XML looked like figure [C]:

      <interface type='network'>
        <source network='testnet' portgroup='admin'/>
        <target dev='macvtap0'/>
        <alias name='net0'/>
        <address type='pci' domain='0x0000' bus='0x00'
                 slot='0x03' function='0x0'/>
      </interface>

and after the change, it looked like figure [E]:

      <interface type='direct'>
        <source dev='p4p1_0' mode='bridge'/>
        <bandwidth>
            <inbound average='1000' peak='5000' burst='1024'/>
            <outbound average='128' peak='256' burst='256'/>
        </bandwidth>
        <target dev='macvtap0'/>
        <alias name='net0'/>
        <address type='pci' domain='0x0000' bus='0x00'
                 slot='0x03' function='0x0'/>
      </interface>

You'll notice that bandwidth info, physdev, and macvtap mode have been
added, but the network and portgroup names are now missing - I didn't
think that this information was of any use once the needed
bandwidth/vlan/etc config had been pulled from the network/portgroup.

I was wrong.

A few months after that change a user on IRC asked what happened to
portgroup in the status XML and described how he used it (more or less
as a tag to decide what external information to use in a hook script
that was run at startup/migration time - see
http://wiki.libvirt.org/page/OVS_and_PVLANS ). At that time I planned
to make a patch to re-add portgroup, but life intervened as that was
just prior to a transatlantic move involving several weeks of
"vacation". During this time I somehow forgot to make the patch, and
also mistakenly remembered that I *had* made it.

Subsequent to this, as a part of mprivozn's work to add support for
network-specific hooks, I did re-add the output of the network name in
status XML, but once again completely forgot about portgroup. This was
in commit a3609121 (first appearing in libvirt 1.2.11). This made the
status XML from the above example look like this:

      <interface type='direct'>
        <source network='testnet' dev='p4p1_0' mode='bridge'/>
        <bandwidth>
            <inbound average='1000' peak='5000' burst='1024'/>
            <outbound average='128' peak='256' burst='256'/>
        </bandwidth>
        <target dev='macvtap0'/>
        <alias name='net0'/>
        <address type='pci' domain='0x0000' bus='0x00'
                 slot='0x03' function='0x0'/>
      </interface>

*This* patch just adds the portgroup back to the status XML, so the
 same example interface will look like this:

      <interface type='direct'>
        <source network='testnet' portgroup='admin'
                dev='p4p1_0' mode='bridge'/>
        <bandwidth>
            <inbound average='1000' peak='5000' burst='1024'/>
            <outbound average='128' peak='256' burst='256'/>
        </bandwidth>
        <target dev='macvtap0'/>
        <alias name='net0'/>
        <address type='pci' domain='0x0000' bus='0x00'
                 slot='0x03' function='0x0'/>
      </interface>

The result is that the status XML now contains all information about
how the interface is setup (bandwidth, physical device, tap device,
etc), in addition to pointers to its origin (the network and
portgroup).