docs: Convert 'drvxen' page to rST

Fix the referenced anchor in 'formatdomain.rst' right away.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Erik Skultety <eskultet@redhat.com>

docs/meson.build

@@ -22,7 +22,6 @@ docs_html_in_files = [
   'csharp',
   'dbus',
   'docs',
-  'drvxen',
   'firewall',
   'format',
   'formatcaps',
@@ -81,6 +80,7 @@ docs_rst_files = [
   'drvvbox',
   'drvvirtuozzo',
   'drvvmware',
+  'drvxen',
   'errors',
   'formatbackup',
   'formatcheckpoint',