Start by dumping the xml for the domain to somewhere you can grab it again later:

[root@kvmhost ~]# virsh dumpxml guest1 > /var/tmp/guest1.xml

Make sure the domain is not persistent:

[root@kvmhost ~]# virsh undefine guest1
Domain guest1 has been undefined

The actual migration (you may wish to check that you’re not overwriting the target, highlighted in red):

[root@kvmhost ~]# virsh blockcopy guest1 /kvm/guest1.img /var/lib/libvirt/images/storagehost-NFS4/guest1.img --wait --verbose --pivot
Block Copy: [100 %]
Successfully pivoted

Redefine the domain, double check it:

[root@kvmhost ~]# virsh define /var/tmp/guest1.xml
Domain guest1 defined from /var/tmp/guest1.xml

[root@kvmhost ~]# virsh dominfo guest1
Id: 9
Name: guest1
UUID: e440e5bd-6d71-4807-a276-11477b764751
OS Type: hvm
State: running
CPU(s): 1
CPU time: 438.6s
Max memory: 3145728 KiB
Used memory: 3145728 KiB
Persistent: yes
Autostart: disable
Managed save: no
Security model: none
Security DOI: 0

Verify that the source file is no longer the disk in use and erase the file (if you want):

[root@kvmhost ~]# lsof | grep /kvm/guest1.img
[root@kvmhost ~]# rm /kvm/guest1.img
rm: remove regular file ‘/kvm/guest1.img’? y

Done