vhost-user-gpu: Release memory returned by vu_queue_pop() with free()

vu_queue_pop() returns memory that must be freed with free().

Cc: qemu-stable@nongnu.org
Reported-by: Coverity (CID 1421887 ALLOC_FREE_MISMATCH)
Suggested-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>