block/qed: Fix missing ERRP_GUARD() for error_prepend()

As the comment in qapi/error, passing @errp to error_prepend() requires
ERRP_GUARD():

* = Why, when and how to use ERRP_GUARD() =
*
* Without ERRP_GUARD(), use of the @errp parameter is restricted:
...
* - It should not be passed to error_prepend(), error_vprepend() or
*   error_append_hint(), because that doesn't work with &error_fatal.
* ERRP_GUARD() lifts these restrictions.
*
* To use ERRP_GUARD(), add it right at the beginning of the function.
* @errp can then be used without worrying about the argument being
* NULL or &error_fatal.

ERRP_GUARD() could avoid the case when @errp is &error_fatal, the user
can't see this additional information, because exit() happens in
error_setg earlier than information is added [1].

The bdrv_qed_co_invalidate_cache() passes @errp to error_prepend()
without ERRP_GUARD().

Though it is a BlockDriver.bdrv_co_invalidate_cache() method, and
currently its @errp parameter only points to callers' local_err, to
follow the requirement of @errp, add missing ERRP_GUARD() at the
beginning of this function.

[1]: Issue description in the commit message of commit ae7c80a7bd
     ("error: New macro ERRP_GUARD()").

Cc: Stefan Hajnoczi <stefanha@redhat.com>
Cc: Kevin Wolf <kwolf@redhat.com>
Cc: Hanna Reitz <hreitz@redhat.com>
Cc: qemu-block@nongnu.org
Signed-off-by: Zhao Liu <zhao1.liu@intel.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-ID: <20240311033822.3142585-10-zhao1.liu@linux.intel.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>

block/qed.c

@@ -1579,6 +1579,7 @@ bdrv_qed_co_change_backing_file(BlockDriverState *bs, const char *backing_file,
 static void coroutine_fn GRAPH_RDLOCK
 bdrv_qed_co_invalidate_cache(BlockDriverState *bs, Error **errp)
 {
+    ERRP_GUARD();
     BDRVQEDState *s = bs->opaque;
     int ret;