Qemu-kvm: information leak in virtio devices

A flaw was found in QEMU, in the virtio-scsi, virtio-blk, and virtio-crypto devices. The size for virtqueue_push as set in virtio_scsi_complete_req / virtio_blk_req_complete / virito_crypto_req_complete could be larger than the true size of the data which has been sent to guest. Once virtqueue_push() finally calls dma_memory_unmap to ummap the in_iov, it may call the address_space_write function to write back the data. Some uninitialized data may exist in the bounce.buffer, leading to an information leak.