vnc: -readconfig fix

Now that -vnc goes through QemuOpts we can get vnc configuration
via -readconfig too.  So setting display_remote in the command
line parsing code doesn't cut it any more, we must check QemuOpts
instead to see whenever any vnc display is configured.

Reported-by: Markus Armbruster <armbru@redhat.com>
Tested-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
