Be aware that *libvirt* starts a *new* instance of `dnsmasq` for each virtual network (click for more)
This will very likely cause conflicts with Pi-hole’s dnsmasq instance if both instances try to bind the same network interface.
By default, dnsmasq will try to bind all addresses, even if configured to listen only on specific interfaces - it just discards requests from unconfigured interfaces.
To avoid such conflicts, you have to force dnsmasq to bind only the very interfaces it is listening to. You can do that via the bind-interfaces option within dnsmasq.
Whether you should apply this to libvrt’s dnsmasq or Pi-hole’s or both and what interfaces would go where will likely depend on your specfific configuration needs.
I have come across a similar issue a while ago, slightly differing by involving NetworkManager and a KVM virtual machine in addition to libvirt, but the solution may be relevant here as well - have a look at deHakkelaar's solution for Can't get pihole to work on br0 interface - #18 by deHakkelaar.
EDIT: (short summary: OP did reconfigure bridge interface to static, deinstalled dnsmasq and removed left-over references to bind-interfaces.)
In addition, with regards to your error, make sure that systemd-resolved is up and running.