LXC configuration can contain lines like: lxc.network.ip[46] = some.ip.addr.ess LXC then sets this IP address to the guest network device. Supporting a similar feature in libvirt would help fill that feature gap.
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Actually, according to https://libvirt.org/formatdomain.html#ip-configuration this capability has been supported since libvirt 1.2.12 (Commit c9a641f1e, which was written by Cedric himself), which was released all the way back in January 2015. There were some subsequent additions made in libvirt 2.1.0 to support setting the IP address of the *host* side of the beth pair, as well as adding static routes on both the guest and host side. So this feature has been in libvirt for a very long time, but nobody noticed this BZ requesting it :-)