hw/virtio/vhost: 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].

In hw/virtio/vhost.c, there are 2 functions passing @errp to
error_prepend() without ERRP_GUARD():
- vhost_save_backend_state()
- vhost_load_backend_state()

Their @errp both points to callers' @local_err. However, as the APIs
defined in include/hw/virtio/vhost.h, it is necessary to protect their
@errp with ERRP_GUARD().

To follow the requirement of @errp, add missing ERRP_GUARD() at their
beginning.

[1]: Issue description in the commit message of commit ae7c80a
     ("error: New macro ERRP_GUARD()").

Cc: "Michael S. Tsirkin" <mst@redhat.com>
Signed-off-by: Zhao Liu <zhao1.liu@intel.com>
Message-ID: <20240311033822.3142585-27-zhao1.liu@linux.intel.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>

hw/virtio/vhost.c

@@ -2199,6 +2199,7 @@ int vhost_check_device_state(struct vhost_dev *dev, Error **errp)
 
 int vhost_save_backend_state(struct vhost_dev *dev, QEMUFile *f, Error **errp)
 {
+    ERRP_GUARD();
     /* Maximum chunk size in which to transfer the state */
     const size_t chunk_size = 1 * 1024 * 1024;
     g_autofree void *transfer_buf = NULL;
@@ -2291,6 +2292,7 @@ int vhost_save_backend_state(struct vhost_dev *dev, QEMUFile *f, Error **errp)
 
 int vhost_load_backend_state(struct vhost_dev *dev, QEMUFile *f, Error **errp)
 {
+    ERRP_GUARD();
     size_t transfer_buf_size = 0;
     g_autofree void *transfer_buf = NULL;
     g_autoptr(GError) g_err = NULL;