libvirtd: fix IPv6 on direct NIC

Set the trustGuestRxFilters attribute on the NIC to yes.

<interface type='direct' trustGuestRxFilters='yes'>
    <mac address='52:54:14:e1:18:f3'/>
    <source dev='eth0' mode='bridge'/>
    <model type='virtio'/>
    <address type='pci' domain='0x0000' bus='0x04' slot='0x00' function='0x0'/>                                                  </interface>

Reference

https://libvirt.org/formatdomain.html#direct-attachment-to-physical-interface