maint: improve VIR_ERR_INVALID_SECRET usage

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>