libvirt users - Jul 2016 - Backups failing due to snapshot already exists

My VMs are raw disk images as logical volumes. My backup script takes a
snapshot of a VM's volume, copies it off to another machine and then
deletes the snapshot.

For some reason I have a snapshot that didn't delete which is now
preventing the backups from proceeding:

...
Friday 1 July  02:10:01 AEST 2016: Taking snapshot of
/dev/vms-lvm/MBE3-MythUbu-14.04-64bit.raw...
ERR: The specified snapshot volume
/dev/vms-lvm/MBE3-MythUbu-14.04-64bit.snap already exists. Operation
canceled.
...

But if I try to remove the snapshot volume I get an error that it's in use:

$ sudo lvremove /dev/vms-lvm/MBE3-MythUbu-14.04-64bit.snap
  Logical volume vms-lvm/MBE3-MythUbu-14.04-64bit.snap in use.

Can anyone please advise on how I find out what's using the snapshot, then
kill that process so I can remove the snapshot so my backups can work again?