O_DIRECT on Linux has alignment requirements on I/O buffers and
misaligned requests result in -EINVAL.  The Linux virtio_blk guest
driver usually submits aligned requests so I forgot to handle misaligned
requests.

It turns out that virtio-win guest drivers submit misaligned requests.
Handle them using a bounce buffer that meets alignment requirements.
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>