VNC pasword authentication support for the paravirt framebuffer server.

VNC pasword authentication support for the paravirt framebuffer server.

The rules for configuring the password are equivalent of those used
for HVM, but the actual guest config option is a little different as a
result of the recent refactoring of the PVFB config file syntax.

 - If the 'vfb' option in the guest config has a 'vncpasswd' parameter
   specified
      - If the passwd is not zero length, use that
      - Else run with no authentication (important as it enables
        override of next rule)
 - Else-if the xend-config.sxp has a password specified use that
 - Else run with no authentication

Example configuration:

 - To set an explicit guest password:
    vfb = [ "type=vnc,vncunused=1,vnclisten=0.0.0.0,vncpasswd=123456"]

 - To disable authentication, overriding any XenD configured
   default password
    vfb = [ "type=vnc,vncunused=1,vnclisten=0.0.0.0,vncpasswd="]

 - To run with default XenD configured password (if any)
    vfb = [ "type=vnc,vncunused=1,vnclisten=0.0.0.0"]

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