blockjobs: fix set-speed kick

If speed is '0' it's not actually "less than" the previous speed.
Kick the job in this case too.
Signed-off-by: NJohn Snow <jsnow@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Reviewed-by: NKevin Wolf <kwolf@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

blockjob.c

@@ -499,7 +499,7 @@ void block_job_set_speed(BlockJob *job, int64_t speed, Error **errp)
     }
 
     job->speed = speed;
-    if (speed <= old_speed) {
+    if (speed && speed <= old_speed) {
         return;
     }