xl: remove parsing of "vncviewer" option in xl domain config file

xl: remove parsing of "vncviewer" option in xl domain config file

Print out a warning and suggest user use "-V" option when invoking "xl
create". Also remove that option in manpage. This will introduce a
minor functional regression but it's very easy to work around.

The rationale behind this change is that, this option is actually not
part of domain configuration. It just affects whether a vncviewer
should be automatically spawn, but has nothing to do with how a domain
should be constructed. And this option is also bogus, considering if you
migrate a domain to a remote host and the receiver spawns a vncviewer on
the receiving side then it either dies silently or occupies resource.

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>