1. login as root

2. change configuration

# virsh dumpxml guestname  > /root/guestname.xml  

(backup configuration)

# vim /root/guestname.xml 

(change configuration)

# virsh define /root/guestname.xml 

(enable changes)
3. shutdown guest

# virsh shutdown guest

or with virt-manager

4. start guest

# virsh start guestvm