[HVM/vncserver] Implement a 'vnclisten' option to limit the interface

[HVM/vncserver] Implement a 'vnclisten' option to limit the interface
that the VNC server from qemu listens on.

Defaults to only listen on 127.0.0.1

The old behaviour (listen on all interfaces) can be restored, by
- changing the system-wide default in /etc/xen/xend-config.sxp by adding:
(vnc-listen '0.0.0.0')
- changing individual domain config files by adding:
vnclisten="0.0.0.0"

Also allows specifying the hostname associated with an interface to limit
to that interface.

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