mirror: fix misleading comments

s/target bs/to_replace/, also we check to_replace bs is not
blocked in qmp_drive_mirror() not here

Signed-off-by: Changlong Xie <xiecl.fnst@cn.fujitsu.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Jeff Cody <jcody@redhat.com>
Message-id: 1466672241-22485-3-git-send-email-xiecl.fnst@cn.fujitsu.com
Signed-off-by: Jeff Cody <jcody@redhat.com>
