remote: fix typo in remoteDomainOpenGraphicsFD
    
    The remoteDomainOpenGraphicsFD method was using the wrong RPC
    arg struct remote_domain_open_graphics_args instead of
    remote_domain_open_graphics_fd_args. Fortunately both structs
    had identical contents so there was no functional bug, but to
    avoid confusing future maintainers, we should fix it.
    
    Signed-off-by: Daniel P. Berrange <berrange@redhat.com>