blockdev: Fix blockdev-snapshot-sync error reporting for no medium

When external_snapshot_abort() rejects a BlockDriverState without a
medium, it creates an error like this:

        error_setg(errp, "Device '%s' has no medium", device);

Trouble is @device can be null.  My system formats null as "(null)",
but other systems might crash.  Reproducer:

1. Create a block device without a medium

    -> {"execute": "blockdev-add", "arguments": {"driver": "host_cdrom", "node-name": "blk0", "filename": "/dev/sr0"}}
    <- {"return": {}}

3. Attempt to snapshot it

    -> {"execute":"blockdev-snapshot-sync", "arguments": { "node-name": "blk0", "snapshot-file":"/tmp/foo.qcow2","format":"qcow2"}}
    <- {"error": {"class": "GenericError", "desc": "Device '(null)' has no medium"}}

Broken when commit 0901f67ecd made @device optional.

Use bdrv_get_device_or_node_name() instead.  Now it fails as it
should:

    <- {"error": {"class": "GenericError", "desc": "Device 'blk0' has no medium"}}

Fixes: 0901f67ecd ("qmp: Allow to take external snapshots on bs graphs node.")
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-ID: <20240306142831.2514431-1-armbru@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

blockdev.c

@@ -1395,7 +1395,8 @@ static void external_snapshot_action(TransactionAction *action,
     bdrv_drained_begin(state->old_bs);
 
     if (!bdrv_is_inserted(state->old_bs)) {
-        error_setg(errp, QERR_DEVICE_HAS_NO_MEDIUM, device);
+        error_setg(errp, QERR_DEVICE_HAS_NO_MEDIUM,
+                   bdrv_get_device_or_node_name(state->old_bs));
         return;
     }