vnc: allow to connect with add_client when -vnc none

Do not skip VNC initialization, in particular of auth method when vnc is
configured without sockets, since we should still allow connections
through QMP add_client.

Fixes:
https://bugzilla.redhat.com/show_bug.cgi?id=1434551

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-id: 20170328160646.21250-1-marcandre.lureau@redhat.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
