crypto: add block encryption framework

Add a generic framework for supporting different block encryption
formats. Upon instantiating a QCryptoBlock object, it will read
the encryption header and extract the encryption keys. It is
then possible to call methods to encrypt/decrypt data buffers.

There is also a mode whereby it will create/initialize a new
encryption header on a previously unformatted volume.

The initial framework comes with support for the legacy QCow
AES based encryption. This enables code in the QCow driver to
be consolidated later.

Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
