virtio-blk: heap buffer overflow caused by unaligned requests

Multiple heap-based buffer overflows in the virtio-blk driver hw/virtio-blk.c in qemu-kvm 0.14.0 allow local guest users to cause a denial of service guest crash and possibly gain privileges via a 1 write request to the virtioblkhandlewrite function or 2 read request to the virtioblkhandleread...