With gcrypt, most of the dispatch happens in the library,
so there aren't many classes to create.  However, we can
still create separate dispatch for CTR mode, and for
CONFIG_QEMU_PRIVATE_XTS, which avoids needing to check
for these modes at runtime.

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