tablet: fix unexpected termination issue

tablet: fix unexpected termination issue

'Null Pointer Dereference' occurs, the emulator dies unexpectedly.
It is caused by commit f998ca30b51b5c549898763d4bd5041380a60416.
The committer have made a mistake when applying the virtio API modification.
The virtio API has been changed as follows:
    previous: int virtqueue_pop(VirtQueue *vq, VirtQueueElement *elem)
    current: void *virtqueue_pop(VirtQueue *vq, size_t sz)

Change-Id: I2c5a20c2741633f855ab0128f7e4dd989404b2b2
Signed-off-by: Jinhyung Jo <jinhyung.jo@samsung.com>