ui/vnc: defer listener registration until the console is known

Previously, the display change listener was registered early in
vnc_display_new() without a console, requiring vnc_display_open() to
conditionally unregister and re-register it when the actual console was
resolved. Since vnc_display_new() and vnc_display_open() were merged in
the previous commit, simply delay the registration and keyboard state
initialization to vnc_display_open(), after the console has been looked
up. This removes the conditional re-registration and simplifies the code.

Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>