error: Move ERRP_GUARD() to the beginning of the function

include/qapi/error.h advises to put ERRP_GUARD() right at the
beginning of the function, because only then can it guard the whole
function.  Clean up the few spots disregarding the advice.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20221121085054.683122-4-armbru@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>