ui/console: move QemuDmaBuf struct def to dmabuf.c

ui/console: move QemuDmaBuf struct def to dmabuf.c

To complete privatizing process of QemuDmaBuf, QemuDmaBuf struct def
is moved to dmabuf.c

Suggested-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Cc: Philippe Mathieu-Daudé <philmd@linaro.org>
Cc: Daniel P. Berrangé <berrange@redhat.com>
Cc: Vivek Kasireddy <vivek.kasireddy@intel.com>
Signed-off-by: Dongwon Kim <dongwon.kim@intel.com>
Message-Id: <20240508175403.3399895-7-dongwon.kim@intel.com>