secret: Create a 'base64File' in virSecretObj

secret: Create a 'base64File' in virSecretObj

This patch removes need for secretBase64Path and secretComputePath. Similar
to the configFile, create an entry for base64File, which will be generated
as the driver->configDir, the UUID value, plus the ".base" suffix. Rather
than generating on the fly, store this in the virSecretObj.

The buildup of the pathname done in loadSecrets where the failure to build
is ignored which is no different than the failure to generate the name
in secretLoadValue which would have been ignored in the failure path
after secretLoad.

This also removes the need for secretComputPath and secretBase64Path.

Signed-off-by: John Ferlan <jferlan@redhat.com>