secret: Introduce virSecretObjGetValue and virSecretObjGetValueSize
    
    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.