xl: 'xl vncviewer' accesses port 0 by any invalid domid

xl: 'xl vncviewer' accesses port 0 by any invalid domid

Currently, with command:
  xl vncviewer invalid_domid
it always brings user to the domU using vncport 5900.
The invalid domid could be an non-existing one or Dom0.
It's better to report error in this case.

Correct libxl_vncviewer_exec:
  In existing code, when vncport is NULL, it still continues
  and will show vncport 5900. So, with 'xl vncviewer 0' it also
  wrongly shows domU using vncport 5900. Correct it to report error
  if vncport is NULL.

Signed-off-by: Chunyan Liu <cyliu@suse.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>