crypto/secret: move main logic from 'secret' to 'secret_common'.

Create base class 'common secret'. Move common data and logic from
'secret' to 'common_secret' class. This allowed adding abstraction layer
for easier adding new 'secret' objects in future.
Convert 'secret' class to child from basic 'secret_common' with 'data'
and 'file' properties.

Signed-off-by: Alexey Krasikov <alex-krasikov@yandex-team.ru>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>

crypto/Makefile.objs

@@ -18,6 +18,7 @@ crypto-obj-y += tlscredsanon.o
 crypto-obj-y += tlscredspsk.o
 crypto-obj-y += tlscredsx509.o
 crypto-obj-y += tlssession.o
+crypto-obj-y += secret_common.o
 crypto-obj-y += secret.o
 crypto-obj-y += pbkdf.o
 crypto-obj-$(CONFIG_NETTLE) += pbkdf-nettle.o