vm-hosts: Fix domain label

The `__path__` label is automatically changed to `filename` before the
processing pipeline begins.

group_vars/vm-hosts.yml

@@ -275,7 +275,7 @@ promtail_scrape_configs:
       job: vm-console
   pipeline_stages:
   - regex:
-      source: __path__
+      source: filename
       expression: '/var/log/libvirt/console/(?P<domain>.*)\.log$'
   - labels:
       domain:
@@ -286,7 +286,7 @@ promtail_scrape_configs:
       job: qemu
   pipeline_stages:
   - regex:
-      source: __path__
+      source: filename
       expression: '.*/(?P<domain>.*)\.log$'
   - labels:
       domain: