docs: define semantics of vncpasswd in xl.cfg

docs: define semantics of vncpasswd in xl.cfg

A recent discussion around LSN-2016-0001 [1] included defining
the sematics of an empty string for a VNC password. It was stated
that "libxl interprets an empty password in the caller's
configuration to mean that passwordless access should be permitted".

The same applies for vncpasswd setting in xl.cfg. This patch
extends to xl.cfg documentation to define the semantics of setting
vncpasswd to an empty string.

Signed-off-by: Jim Fehlig <jfehlig@suse.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>