nixos.libvirtd: fix broken VMs due to emulator path changes

This had already been fixed in f52f9bf,
but the problem was reintroduced in
bce59a1 because the path to the XML
files changed.

(cherry picked from commit af01fa7)

nixos/modules/virtualisation/libvirtd.nix

@@ -123,7 +123,7 @@ in {
         # config file. But this path can unfortunately be garbage collected
         # while still being used by the virtual machine. So update the
         # emulator path on each startup to something valid (re-scan $PATH).
-        for file in /etc/libvirt/qemu/*.xml /etc/libvirt/lxc/*.xml; do
+        for file in /var/lib/libvirt/qemu/*.xml /var/lib/libvirt/lxc/*.xml; do
             test -f "$file" || continue
             # get (old) emulator path from config file
             emulator=$(grep "^[[:space:]]*<emulator>" "$file" | sed 's,^[[:space:]]*<emulator>\(.*\)</emulator>.*,\1,')