nix: include libvirt in cross-version migration testing

Adds support for `libvirt` in cross-version migration testing introduced in cyberus-technology/libvirt-tests#196.

The change in `libvirt-tests` moved `libvirt` from a parameter to `pkgs`.
This commit adapts the override to use the `libvirt` version from the repo to match the change.

On-behalf-of: SAP julian.schindel@sap.com
Signed-off-by: Julian Schindel <julian.schindel@cyberus-technology.de>

Author: arctic-alpaca