vhost-user-gpu: use an extandable state enum for commands

Introduce a pending state for commands which aren't finished yet, but
are being handled. See following patch.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20210204105232.834642-4-marcandre.lureau@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>