Regarding IPv6, there is support. In fact I run IPv6 only networks including for the hypervisors.

    $ virsh net-dumpxml default6
    <network>
      <name>default6</name>
      <uuid>73590ea2-eb15-4e67-b104-319721bdf302</uuid>
      <forward mode='route'/>
      <bridge name='virbr1' stp='on' delay='0'/>
      <mac address='52:54:00:ff:a7:2d'/>
      <domain name='default6'/>
      <ip family='ipv6' address='2001:db8:ffff::1' prefix='48'>
      </ip>
    </network>

One can also use DHCPv6 if required.