xen_disk: Account for failed and invalid operations

Signed-off-by: Alberto Garcia <berto@igalia.com>
Message-id: e0cbb96cb0e1f86c37c7ce332efdf02b57b9d365.1446044838.git.berto@igalia.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
