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 <[email protected]>
Reviewed-by: Fam Zheng <[email protected]>
Signed-off-by: Daniel P. Berrange <[email protected]>