crypto: add CTR mode support

crypto: add CTR mode support

Introduce CTR mode support for the cipher APIs.
CTR mode uses a counter rather than a traditional IV.
The counter has additional properties, including a nonce
and initial counter block. We reuse the ctx->iv as
the counter for conveniences.

Both libgcrypt and nettle are support CTR mode, the
cipher-builtin doesn't support yet.

Signed-off-by: Gonglei <[email protected]>
Signed-off-by: Daniel P. Berrange <[email protected]>