crypto/gcrypt: Split QCryptoCipherGcrypt into subclasses

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>