nvme: Account for failed and invalid operations

Signed-off-by: Alberto Garcia <berto@igalia.com>
Message-id: 678dc67da229759d404b44f7cc2bf5ed8bf8ad14.1446044838.git.berto@igalia.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
