how to update back-end storage online

Hi, I want to implement a qemu block driver which like qeme/block/iscsi.c. Here are my questions: some targets have be attached to guest system, and then the targets info have changed. How can I update the connection to targets online? I mean the disk in guest system does not change, just update the connection to the back-end storage, and the operation can't affect guest system. I can't find a nova-api or libvirt-api to meet my needs now. Does anyone help me? Thanks.