hw/block/virtio-blk: Clean req->dev repetitions

Some functions sometimes uses req->dev even though a local variable
VirtIOBlock* s = req->dev has already been defined.
Updated places to use s everywhere in the file.

Signed-off-by: Anastasiia Rusakova <arusakova917@gmail.com>
Message-id: 20190307161925.4158-1-rusakova.nastasia@icloud.com
Message-Id: <20190307161925.4158-1-rusakova.nastasia@icloud.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>