linux/block-snapshot: call qvm-template-commit in offline mode

When called from libvirt->libxl, there is libvirt lock taken on that
domain. Because of that, we can't access libvirt domain, so basically
any runtime information. Without that --offline-mode, script waited on
the lock and then was killed by libxl after a timeout - before actually
committing the changes.

linux/system-config/block-snapshot

@@ -245,7 +245,7 @@ case "$command" in
             domain=$(cat "$HOTPLUG_STORE-domain")
             if [ "$domain" ]; then
               # Dont stop on errors
-              /usr/bin/qvm-template-commit "$domain" || true
+              /usr/bin/qvm-template-commit --offline-mode "$domain" || true
             fi
           fi
         fi