Currently the RESTAPI reports the information about the IP addresses of guest network interfaces in several places:

1. /vms/{vm:id}

  <vm href="..." id="...">
    ...
    <name>whatever</name>
    ...
    <guest_info>
      <ips>
        <ip address="192.168.122.30"/>
      </ips>
      <fqdn>whatever.example.com</fqdn>
    </guest_info>
  </vm>

2. /vms/{vm:id}/nics and /vms/{vm:id}/reporteddevices

  <nics>
    <nic>
      ...
      <reported_devices>
        <reported_device href="..." id="..">
          <name>eth0</name>
          <description>guest reported data</description>
          <type>network</type>
          <mac address="00:1a:4a:b5:4c:94"/>
          <ips>
            <ip address="192.168.1.115" version="v4"/>
            <ip address="fe80::21a:4aff:feb5:4c94" version="v6"/>
            <ip address="::1:21a:4aff:feb5:4c94" version="v6"/>
          </ips>
          <vm href="..." id="..."/>
        </reported_device>
      </reported_devices>
      ...
    </nic>
  </nics>

Number 1 was the first implementation, and is kept only for backwards compatibility. It should be removed in version 4.x.