libvirt 9.3 introduced a regression around the default bus of mouse/keyboard devices. More specifically this commit: https://gitlab.com/libvirt/libvirt/-/commit/c4bc4d3b82fbe22e03c986ca896090f481df5c10

Previously when a mouse/keyboard device was added without a bus (for example just <input type="mouse"/>), the bus would default to ps2 upon libvirt's processing of the config. After the commit above, the result is now <input type='mouse' bus='default'/> which is obviously not valid.

This has been fixed upstream via https://gitlab.com/libvirt/libvirt/-/commit/c9056e682a8a67dc29e39eb01392fcf8ee978c31

Ref: https://gitlab.com/libvirt/libvirt/-/issues/577

I have marked this bug with severity "Major" as it can and does block creation/booting of VMs as well as live migrations depending on how you use libvirt.  I'd suggest backporting the fix into EL 9.3 which is where it was first introduced.

Here are some quick steps to recreate it, with the host running EL 9.3, CentOS Stream, Fedora >= 39, or likely any other distro or environment with libvirt >= 9.3.

wget https://repo.almalinux.org/almalinux/9.3/cloud/x86_64/images/AlmaLinux-9-GenericCloud-latest.x86_64.qcow2 -O /var/lib/libvirt/images/AlmaLinux-9-GenericCloud-latest.x86_64.qcow2
virt-install --name libvirtd-bug-alma9 --memory 8192 --vcpus 4 --disk /var/lib/libvirt/images/AlmaLinux-9-GenericCloud-latest.x86_64.qcow2 --import --os-variant almalinux9 --noautoconsole
cat << EOF > mouse.xml
<input type='mouse'/>
EOF
virsh attach-device libvirtd-bug-alma9 --file mouse.xml --current