maint: improve VIR_ERR_INVALID_SECRET usage
    
    While all errors related to invalid secrets appeared to be
    consistent, we might as well continue the trend of using a
    common macro.  Just as in commit 6e130ddc, the difference
    between VIR_IS_SECRET and VIR_IS_CONNECTED_SECRET is moot
    (due to reference counting, any valid secret must be tied to
    a valid domain).  For now, we don't need virCheckSecretGoto().
    
    * src/datatypes.h (virCheckSecretReturn): New macro.
    (VIR_IS_SECRET, VIR_IS_CONNECTED_SECRET): Drop unused macros.
    * src/libvirt.c: Use macro throughout.
    (virLibSecretError): Drop unused macro.
    
    Signed-off-by: Eric Blake <eblake@redhat.com>