atapi: Account for failed and invalid operations

Signed-off-by: Alberto Garcia <berto@igalia.com>
Message-id: 59dee4e2921b0c79d41c49b67dfb93d32db9f7f9.1446044838.git.berto@igalia.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
