block/qcow2: simplify qcow2_co_invalidate_cache()

block/qcow2: simplify qcow2_co_invalidate_cache()

qcow2_do_open correctly sets errp on each failure path. So, we can
simplify code in qcow2_co_invalidate_cache() and drop explicit error
propagation.

Add ERRP_GUARD() as mandated by the documentation in
include/qapi/error.h so that error_prepend() is actually called even if
errp is &error_fatal.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Alberto Garcia <berto@igalia.com>
Reviewed-by: Greg Kurz <groug@kaod.org>
Message-Id: <20210202124956.63146-13-vsementsov@virtuozzo.com>
Signed-off-by: Eric Blake <eblake@redhat.com>