secret: Alter virSecretGetSecretString

secret: Alter virSecretGetSecretString

Rather than returning a "char *" indicating perhaps some sized set of
characters that is NUL terminated, alter the function to return 0 or -1
for success/failure and add two parameters to handle returning the
buffer and it's size.

The function no longer encodes the returned secret, rather it returns
the unencoded secret forcing callers to make the necessary adjustments.

Alter the callers to handle the adjusted model.

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