backup: Improve error for bdrv_getlength() failure

bdrv_get_device_name() will be an empty string with modern management
tools that don't use -drive. Use bdrv_get_device_or_node_name() instead
so that the node name is used if the BlockBackend is anonymous.

While at it, start with upper case to make the message consistent with
the rest of the function.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Alberto Garcia <berto@igalia.com>
Message-Id: <20200430142755.315494-3-kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

block/backup.c

@@ -400,8 +400,8 @@ BlockJob *backup_job_create(const char *job_id, BlockDriverState *bs,
 
     len = bdrv_getlength(bs);
     if (len < 0) {
-        error_setg_errno(errp, -len, "unable to get length for '%s'",
-                         bdrv_get_device_name(bs));
+        error_setg_errno(errp, -len, "Unable to get length for '%s'",
+                         bdrv_get_device_or_node_name(bs));
         goto error;
     }