The following config elements now support a <vlan> subelements:

within a domain: <interface>, and the <actual> subelement of <interface>
within a network: the toplevel, as well as any <portgroup>

Each vlan element must have one or more <tag id='n'/> subelements.  If
there is more than one tag, it is assumed that vlan trunking is being
requested. If trunking is required with only a single tag, the
attribute "trunk='yes'" should be added to the toplevel <vlan>
element.

Some examples:

  <interface type='hostdev'/>
    <vlan>
      <tag id='42'/>
    </vlan>
    <mac address='52:54:00:12:34:56'/>
    ...
  </interface>

  <network>
    <name>vlan-net</name>
    <vlan trunk='yes'>
      <tag id='30'/>
    </vlan>
    <virtualport type='openvswitch'/>
  </network>

  <interface type='network'/>
    <source network='vlan-net'/>
    ...
  </interface>

  <network>
    <name>trunk-vlan</name>
    <vlan>
      <tag id='42'/>
      <tag id='43'/>
    </vlan>
    ...
  </network>

  <network>
    <name>multi</name>
    ...
    <portgroup name='production'/>
      <vlan>
        <tag id='42'/>
      </vlan>
    </portgroup>
    <portgroup name='test'/>
      <vlan>
        <tag id='666'/>
      </vlan>
    </portgroup>
  </network>

  <interface type='network'/>
    <source network='multi' portgroup='test'/>
    ...
  </interface>

IMPORTANT NOTE: As of this patch there is no backend support for the
vlan element for *any* network device type. When support is added in
later patches, it will only be for those select network types that
support setting up a vlan on the host side, without the guest's
involvement. (For example, it will be possible to configure a vlan for
a guest connected to an openvswitch bridge, but it won't be possible
to do that for one that is connected to a standard Linux host bridge.)