crypto: add support for generating initialization vectors

crypto: add support for generating initialization vectors

There are a number of different algorithms that can be used
to generate initialization vectors for disk encryption. This
introduces a simple internal QCryptoBlockIV object to provide
a consistent internal API to the different algorithms. The
initially implemented algorithms are 'plain', 'plain64' and
'essiv', each matching the same named algorithm provided
by the Linux kernel dm-crypt driver.

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