conf: simplify internal virSecretDef handling of usage

conf: simplify internal virSecretDef handling of usage

The public virSecret object has a single "usage_id" field
but the virSecretDef object has a different 'char *' field
for each usage type, but the code all assumes every usage
type has a corresponding single string. Get rid of the
pointless union in virSecretDef and just use "usage_id"
everywhere. This doesn't impact public XML format, only
the internal handling.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>