secret: Introduce virSecretObjListAdd* and virSecretObjListRemove

secret: Introduce virSecretObjListAdd* and virSecretObjListRemove

Add the functions to add/remove elements from the hashed secret obj list.
These will replace secret_driver functions secretAssignDef and secretObjRemove.

The virSecretObjListAddLocked will perform the necessary lookups and
decide whether to replace an existing hash entry or create a new one.
This includes setting up the configPath and base64Path as well as being
able to support the caller's need to restore from a previous definition
in case something goes wrong in the caller.