Introduce the final accessor's to _virSecretObject data and move the
structure from virsecretobj.h to virsecretobj.c

The virSecretObjSetValue logic will handle setting both the secret
value and the value_size. Some slight adjustments to the error path
over what was in secretSetValue were made.

Additionally, a slight logic change in secretGetValue where we'll
check for the internalFlags and error out before checking for
and erroring out for a NULL secret->value. That way, it won't be
obvious to anyone that the secret value wasn't set rather they'll
just know they cannot get the secret value since it's private.