qcow: document another weakness of qcow AES encryption

qcow: document another weakness of qcow AES encryption

Document that use of guest virtual sector numbers as the basis for
the initialization vectors is a potential weakness, when combined
with internal snapshots or multiple images using the same passphrase.
This fixes the formatting of the itemized list too.

Reviewed-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Alberto Garcia <berto@igalia.com>
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Message-id: 20170623162419.26068-4-berrange@redhat.com
Signed-off-by: Max Reitz <mreitz@redhat.com>