block/virtio-blk: 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 virtio_blk_vq_aio_context_init() passes @errp to error_prepend().

Though its @errp points its caller's local @err variable, to follow the
requirement of @errp, add missing ERRP_GUARD() at the beginning of
virtio_blk_vq_aio_context_init().

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

Cc: Stefan Hajnoczi <stefanha@redhat.com>
Cc: "Michael S. Tsirkin" <mst@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>
Acked-by: "Michael S. Tsirkin" <mst@redhat.com>
Message-ID: <20240311033822.3142585-14-zhao1.liu@linux.intel.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>

hw/block/virtio-blk.c

@@ -1682,6 +1682,7 @@ static bool apply_iothread_vq_mapping(
 /* Context: BQL held */
 static bool virtio_blk_vq_aio_context_init(VirtIOBlock *s, Error **errp)
 {
+    ERRP_GUARD();
     VirtIODevice *vdev = VIRTIO_DEVICE(s);
     VirtIOBlkConf *conf = &s->conf;
     BusState *qbus = BUS(qdev_get_parent_bus(DEVICE(vdev)));