docs: drvqemu: Add note about deprecation of domxml-from-native

Add a warning that the operation will no longer work.
Signed-off-by: NPeter Krempa <pkrempa@redhat.com>
Reviewed-by: NJán Tomko <jtomko@redhat.com>

docs/drvqemu.html.in

@@ -426,6 +426,10 @@ mount -t cgroup none /dev/cgroup -o devices
 
     <h3><a id="xmlimport">Converting from QEMU args to domain XML</a></h3>
 
+    <p>
+      <b>Note:</b> this operation is <span class="deprecated"> deprecated as of
+        5.5.0</span> and will return an error.
+    </p>
     <p>
       The <code>virsh domxml-from-native</code> provides a way to
       convert an existing set of QEMU args into a guest description