remote-viewer-connect: centralize window
    
    Instead of top-left corner.
    
    Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1508274
    
    Signed-off-by: Victor Toso <victortoso@redhat.com>
    Acked-by: Eduardo Lima (Etrunko) <etrunko@redhat.com>