virtio-blk: Convert VirtIOBlockReq.elem to pointer

This will make converging with dataplane code easier.

Add virtio_blk_free_request to handle the freeing of request internal
fields.

Signed-off-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>