Use separate classes for each cipher entry point: des_rfb, des3,
aes128, aes192, aes256, cast128, serpent, and twofish.

Generate wrappers for XTS only for CONFIG_QEMU_PRIVATE_XTS.
This eliminates unreachable wrappers for DES_RFB, DES3 and
CAST128, which have blocksizes that do not allow XTS mode.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>