qemu-io: Account for failed, invalid and flush operations

Signed-off-by: Alberto Garcia <berto@igalia.com>
Message-id: 78a7662a8636e55991737ece50003a2dc5a5f3e0.1446044838.git.berto@igalia.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
