virtio-blk: Account for failed and invalid operations

Signed-off-by: Alberto Garcia <berto@igalia.com>
Message-id: 4f623ce52c9d673d35a043fc2959526b41b685c6.1446044838.git.berto@igalia.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
