qemu vnc auth 4/4: XenD config for VNC TLS protocol

qemu vnc auth 4/4: XenD config for VNC TLS protocol

This patch adds support to XenD for configuring the previously added
TLS encryption and x509 certificate validation. At this time I have
only enabled this config to be done system-wide via
/etc/xen/xend-config.sxp. Since it requires the admin to add
certificates on the local FS, there's not much point in making it per
VM. The x509 certificates are located in /etc/xen/vnc. Since this
requires a special VNC client program (GTK-VNC,
virt-viewer/virt-manager or VeNCrypt viewer) the use of TLS is
disabled by default. Admins can enable it if they are using a suitable
client.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>